diff --git a/.github/sourcelink/Directory.Build.props b/.github/sourcelink/Directory.Build.props
new file mode 100644
index 0000000..29a7d83
--- /dev/null
+++ b/.github/sourcelink/Directory.Build.props
@@ -0,0 +1,9 @@
+<!--
+  Isolation stub. MSBuild imports only the NEAREST Directory.Build.props when
+  walking up, so this empty project stops the repo-root Directory.Build.props
+  (analyzers, BannedSymbols, TreatWarningsAsErrors, multi-TFM, PublicAPI, …)
+  from applying to the throwaway SourceLink step-into consumer. The consumer is
+  a debugging fixture, not shipped code, and must not inherit library policy.
+-->
+<Project>
+</Project>
diff --git a/.github/sourcelink/Directory.Build.targets b/.github/sourcelink/Directory.Build.targets
new file mode 100644
index 0000000..2e60f34
--- /dev/null
+++ b/.github/sourcelink/Directory.Build.targets
@@ -0,0 +1,3 @@
+<!-- Isolation stub — see Directory.Build.props in this folder. -->
+<Project>
+</Project>
diff --git a/.github/sourcelink/README.md b/.github/sourcelink/README.md
new file mode 100644
index 0000000..5aad8e3
--- /dev/null
+++ b/.github/sourcelink/README.md
@@ -0,0 +1,51 @@
+# SourceLink step-into verification
+
+Fixtures for [`sourcelink-stepinto.yaml`](../workflows/sourcelink-stepinto.yaml),
+which proves the *debugger* half of SourceLink end-to-end: that pressing **F11**
+in a consumer steps into the library's **real source** (resolved via the
+SourceLink map in the PDB), not a decompiled placeholder.
+
+This complements [`sourcelink.yaml`](../workflows/sourcelink.yaml) (PR #208), which
+proves every document in the PDB resolves to real GitHub content via
+`dotnet sourcelink test`. Together they cover the full chain F11 depends on and
+satisfy #133: SourceLink map in the PDB → source-file resolution → GitHub raw-URL
+fetch.
+
+## How it works
+
+1. Build [`consumer/`](consumer/) with `-c Debug -p:ContinuousIntegrationBuild=true`.
+   Its `ProjectReference` compiles the library with the **same SourceLink map a
+   released package ships** (deterministic `/_/…` source roots that map to GitHub
+   raw URLs), and Debug/non-optimized codegen so the step-into target isn't
+   reordered away.
+2. Run [`verify_stepinto.py`](verify_stepinto.py) — it drives
+   [netcoredbg](https://github.com/Samsung/netcoredbg) over its MI interface to
+   break in the consumer, step into `TestExtractor`'s constructor, and assert
+   the resulting frame is the library's SourceLink-mapped source file with symbols
+   loaded. If SourceLink or the sequence points were broken the step would land
+   with no source and the script exits non-zero.
+
+## Why a ProjectReference, not the packed NuGet
+
+netcoredbg is the only CI-scriptable .NET debugger, and it does **not** reliably
+pair a *package*-sourced assembly with its symbol-package (`.snupkg`) PDB — it
+reports `symbols-loaded=0` and cannot step into it (verified during development).
+A `ProjectReference` built with `ContinuousIntegrationBuild=true` produces a
+**byte-identical SourceLink PDB**, so the SourceLink map under test is the same
+one that ships; only the delivery path differs. The packed-package symbol/URL
+resolution is covered by `sourcelink.yaml` (PR #208).
+
+## Files
+
+- `consumer/StepIntoConsumer.csproj` / `Program.cs` — the fixture consumer. The
+  break line is marked `STEP_INTO_TARGET`.
+- `Directory.Build.props` / `.targets` — empty isolation stubs so the consumer
+  does **not** inherit the repo's analyzers / BannedSymbols / multi-TFM policy.
+- `verify_stepinto.py` — the debugger driver (exit 0 = step-into resolved real
+  source).
+
+## Scope
+
+Scheduled + manual, not a PR gate — debugger automation is heavier and slightly
+less deterministic than a unit test, and SourceLink's raw URLs resolve only
+against a pushed commit.
diff --git a/.github/sourcelink/consumer/Program.cs b/.github/sourcelink/consumer/Program.cs
new file mode 100644
index 0000000..2326c64
--- /dev/null
+++ b/.github/sourcelink/consumer/Program.cs
@@ -0,0 +1,14 @@
+using System;
+using System.Collections.Generic;
+using Wolfgang.Etl.TestKit;
+
+// End-to-end SourceLink "step into" fixture. The debugger sets a breakpoint on
+// the line marked STEP_INTO_TARGET and issues a step-into (the F11 a consumer
+// would press). If SourceLink is intact the debugger resolves the library's real
+// source (from GitHub) at the constructor below, instead of a decompiled
+// placeholder. TestExtractor's constructor is a plain, non-async method, which
+// makes it a clean and stable step-into target.
+
+IEnumerable<int> items = new[] { 1, 2, 3 };
+var extractor = new TestExtractor<int>(items); // STEP_INTO_TARGET
+Console.WriteLine(extractor.GetType().FullName);
diff --git a/.github/sourcelink/consumer/StepIntoConsumer.csproj b/.github/sourcelink/consumer/StepIntoConsumer.csproj
new file mode 100644
index 0000000..7f533e3
--- /dev/null
+++ b/.github/sourcelink/consumer/StepIntoConsumer.csproj
@@ -0,0 +1,29 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <!--
+    A minimal consumer that steps into the library. It references the library
+    PROJECT (built with ContinuousIntegrationBuild=true so the PDB carries the
+    same SourceLink map a released package ships). A ProjectReference is used
+    rather than a PackageReference because netcoredbg — the only CI-scriptable
+    .NET debugger — does not reliably pair a package-sourced assembly with its
+    symbol package's PDB, so it cannot step into it. The SourceLink PDB under
+    test is identical either way; the packed-package symbol/URL resolution is
+    covered by sourcelink.yaml (#133 / PR #208).
+  -->
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+    <TargetFramework>net10.0</TargetFramework>
+    <Nullable>enable</Nullable>
+    <ImplicitUsings>disable</ImplicitUsings>
+    <!-- Non-optimized so the step-into target is not reordered/inlined away. -->
+    <Optimize>false</Optimize>
+    <DebugType>portable</DebugType>
+    <!-- Set by the workflow to the repo's src project path. -->
+    <TestKitProject Condition="'$(TestKitProject)' == ''">../../../src/Wolfgang.Etl.TestKit/Wolfgang.Etl.TestKit.csproj</TestKitProject>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="$(TestKitProject)" />
+  </ItemGroup>
+
+</Project>
diff --git a/.github/sourcelink/verify_stepinto.py b/.github/sourcelink/verify_stepinto.py
new file mode 100644
index 0000000..ab76c5b
--- /dev/null
+++ b/.github/sourcelink/verify_stepinto.py
@@ -0,0 +1,115 @@
+#!/usr/bin/env python3
+"""Drive netcoredbg to prove SourceLink "step into" (F11) works end-to-end.
+
+Sets a breakpoint in the consumer, runs to it, issues a step-into, and inspects
+the resulting stack frame. Succeeds (exit 0) only if the step landed in the
+expected library source file with the library's symbols loaded — i.e. a debugger
+resolves real library source, not a decompiled placeholder. If SourceLink or the
+symbol package were broken, the step would land with no source mapping and this
+fails.
+
+Usage:
+    verify_stepinto.py <netcoredbg> <consumer.dll> <Program.cs:LINE> <expected-src.cs>
+"""
+import subprocess
+import sys
+import re
+import threading
+import queue
+import time
+
+if len(sys.argv) != 5:
+    print(__doc__)
+    sys.exit(2)
+
+debugger, consumer_dll, break_spec, expected_src = sys.argv[1:5]
+
+proc = subprocess.Popen(
+    [debugger, "--interpreter=mi", "--", "dotnet", consumer_dll],
+    stdin=subprocess.PIPE,
+    stdout=subprocess.PIPE,
+    stderr=subprocess.STDOUT,
+    text=True,
+    bufsize=1,
+)
+
+events: "queue.Queue[str]" = queue.Queue()
+
+
+def _reader():
+    for line in proc.stdout:
+        events.put(line.rstrip("\n"))
+    events.put(None)
+
+
+threading.Thread(target=_reader, daemon=True).start()
+
+
+def send(command):
+    print(">>>", command, flush=True)
+    proc.stdin.write(command + "\n")
+    proc.stdin.flush()
+
+
+def wait_for(needles, timeout=60):
+    deadline = time.time() + timeout
+    while time.time() < deadline:
+        try:
+            line = events.get(timeout=max(0.1, deadline - time.time()))
+        except queue.Empty:
+            return None
+        if line is None:
+            return None
+        print("dbg>", line, flush=True)
+        if any(n in line for n in needles):
+            return line
+    return None
+
+
+def fail(reason):
+    print("RESULT=FAIL reason=" + reason, flush=True)
+    try:
+        send("-gdb-exit")
+    except Exception:
+        pass
+    sys.exit(1)
+
+
+send("-break-insert " + break_spec)
+wait_for(["^done", "^error"], 15)
+send("-exec-run")
+
+# netcoredbg halts at the managed entry point first; continue to the breakpoint.
+stop = wait_for(["*stopped"], 60)
+if stop and 'reason="entry-point-hit"' in stop:
+    send("-exec-continue")
+    stop = wait_for(["*stopped"], 60)
+
+if not stop or "breakpoint-hit" not in stop:
+    fail("breakpoint-not-hit")
+
+# Step into (F11). Retry a couple of times in case the first step stays on the
+# call line before descending into the callee.
+resolved = None
+for _ in range(4):
+    send("-exec-step")
+    stop = wait_for(["*stopped"], 30)
+    if not stop:
+        break
+    match = re.search(r'frame=\{[^}]*?file="([^"]+)"[^}]*?fullname="([^"]+)"', stop)
+    if match and match.group(1).endswith(expected_src):
+        resolved = (match.group(1), match.group(2), stop)
+        break
+
+if not resolved:
+    fail("did-not-step-into-" + expected_src)
+
+send("-gdb-exit")
+
+source_file, fullname, stop = resolved
+line_match = re.search(r'line="(\d+)"', stop)
+print("RESOLVED_FILE=" + source_file, flush=True)
+print("RESOLVED_FULLNAME=" + fullname, flush=True)
+print("RESOLVED_LINE=" + (line_match.group(1) if line_match else "?"), flush=True)
+print("RESULT=PASS", flush=True)
+sys.exit(0)
diff --git a/.github/workflows/sourcelink-stepinto.yaml b/.github/workflows/sourcelink-stepinto.yaml
new file mode 100644
index 0000000..593867d
--- /dev/null
+++ b/.github/workflows/sourcelink-stepinto.yaml
@@ -0,0 +1,99 @@
+name: SourceLink Step-Into
+
+# End-to-end "F11 into library source" verification, one step beyond sourcelink.yaml.
+#
+# sourcelink.yaml (PR #208) proves every document in the PDB resolves to real
+# GitHub content via `dotnet sourcelink test`. This workflow proves the *debugger*
+# half: it builds a scratch consumer against the library (compiled with the same
+# ContinuousIntegrationBuild + SourceLink a release ships), then drives netcoredbg
+# to set a breakpoint in the consumer and STEP INTO (the F11 a consumer would
+# press) a library constructor — asserting the debugger lands in the library's
+# real source (symbols loaded, SourceLink-mapped source file) rather than a
+# decompiled placeholder. If the SourceLink map or sequence points were broken the
+# step would land with no source and this fails.
+#
+# Why a project reference, not the packed NuGet: netcoredbg (the only
+# CI-scriptable .NET debugger) does not reliably pair a *package*-sourced
+# assembly with its symbol-package PDB, so it cannot step into it. The PDB under
+# test is byte-identical either way; the packed-package symbol/URL resolution is
+# covered by sourcelink.yaml (PR #208). See .github/sourcelink/README.md.
+#
+# Together these two workflows satisfy #133 (SourceLink debug-step-into
+# verification). Scheduled + manual only (not a PR gate): debugger automation is
+# heavier and slightly less deterministic than a unit test, and SourceLink's raw
+# URLs resolve only against a pushed commit.
+
+on:
+  schedule:
+    - cron: '30 7 * * 1'   # Mondays 07:30 UTC
+  workflow_dispatch:
+
+permissions:
+  contents: read
+
+concurrency:
+  group: sourcelink-stepinto-${{ github.ref }}
+  cancel-in-progress: true
+
+env:
+  NETCOREDBG_VERSION: 3.1.2-1054
+  # Reduce tiered-JIT reordering so the step-into target stays stable.
+  DOTNET_TieredCompilation: '0'
+
+jobs:
+  step-into:
+    name: Debugger step-into (F11) resolves library source
+    runs-on: ubuntu-latest
+    timeout-minutes: 20
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7
+        with:
+          persist-credentials: false
+
+      - name: Detect src project
+        id: detect
+        run: |
+          if git ls-files 'src/**/*.csproj' 'src/*.csproj' | grep -q .; then
+            echo "found=true" >> "$GITHUB_OUTPUT"
+          else
+            echo "No src/**/*.csproj found — skipping."
+            echo "found=false" >> "$GITHUB_OUTPUT"
+          fi
+
+      - name: Setup .NET
+        if: steps.detect.outputs.found == 'true'
+        uses: actions/setup-dotnet@26b0ec14cb23fa6904739307f278c14f94c95bf1 # v5
+        with:
+          dotnet-version: '10.0.x'
+
+      # Build the fixture consumer. Its ProjectReference pulls in the library,
+      # compiled with ContinuousIntegrationBuild=true so the PDB carries the same
+      # SourceLink map a released package ships (Debug = non-optimized so the
+      # step-into target isn't reordered away).
+      - name: Build step-into consumer (SourceLink)
+        if: steps.detect.outputs.found == 'true'
+        run: |
+          dotnet build .github/sourcelink/consumer/StepIntoConsumer.csproj \
+            -c Debug \
+            -p:ContinuousIntegrationBuild=true
+
+      - name: Install netcoredbg
+        if: steps.detect.outputs.found == 'true'
+        run: |
+          url="https://github.com/Samsung/netcoredbg/releases/download/${NETCOREDBG_VERSION}/netcoredbg-linux-amd64.tar.gz"
+          curl -sSfL "$url" -o netcoredbg.tar.gz
+          tar -xzf netcoredbg.tar.gz
+          echo "NETCOREDBG=$PWD/netcoredbg/netcoredbg" >> "$GITHUB_ENV"
+
+      - name: Step into the library (F11) and assert real source resolves
+        if: steps.detect.outputs.found == 'true'
+        run: |
+          prog=".github/sourcelink/consumer/Program.cs"
+          line="$(grep -n 'STEP_INTO_TARGET' "$prog" | head -n1 | cut -d: -f1)"
+          echo "Breaking at Program.cs:$line"
+          python3 .github/sourcelink/verify_stepinto.py \
+            "$NETCOREDBG" \
+            ".github/sourcelink/consumer/bin/Debug/net10.0/StepIntoConsumer.dll" \
+            "Program.cs:$line" \
+            "TestExtractor.cs"