feat(B-0170.4): seed existence-drift fixture + regression test

tools/substrate-claim-checker/fixtures.test.ts

@@ -13,6 +13,7 @@
 import { describe, expect, test } from "bun:test";
 import { join } from "node:path";
 import { checkFile as checkCounts } from "./check-counts.ts";
+import { checkFile as checkExistence } from "./check-existence.ts";
 
 const fixtures = join(import.meta.dir, "fixtures");
 
@@ -35,3 +36,22 @@ describe("eval-set fixtures / count drift", () => {
     expect(finding.claimIsMinimum).toBe(false);
   });
 });
+
+describe("eval-set fixtures / existence drift", () => {
+  test("existence-drift-missing-doc.md — backtick-quoted path that doesn't exist anywhere is detected", () => {
+    const result = checkExistence(join(fixtures, "existence-drift-missing-doc.md"));
+    expect(result.ok).toBe(true);
+    // Same PR #3611 discipline applied to existence-drift: exact count
+    // and pin the body claim's line so a regression in body-claim
+    // detection cannot be masked by an HTML-comment match. The fixture
+    // path is intentionally synthetic (won't be created accidentally),
+    // so the test stays stable across substrate evolution.
+    expect(result.findings.length).toBe(1);
+    const finding = result.findings[0]!;
+    expect(finding.line).toBe(24);
+    expect(finding.pathClaim).toBe(
+      "docs/_fixture_existence_drift_target_b0170_2026_05_15.md",
+    );
+    expect(finding.severity).toBe("drift");
+  });
+});

tools/substrate-claim-checker/fixtures/README.md

@@ -22,6 +22,7 @@ axis.
 | Sub-class | Fixture | Anchor PR / history | Expected finding |
 |---|---|---|---|
 | count drift | `count-drift-9-vs-15.md` | PR #1259 (`review(pr-1257-postmerge): verify-then-claim count drift (9→18+) frontmatter + body + MEMORY.md`) | "9 drift instances" claim vs 15-row table |
+| existence drift | `existence-drift-missing-doc.md` | PR #1252 — verify-then-claim memo body table instance #8 (`future-domain memo references docs/courier-ferry-protocol.md` / doesn't exist) | backtick-quoted synthetic path resolves at no candidate root |
 
 Add a new row when a new fixture lands.
 

tools/substrate-claim-checker/fixtures/existence-drift-missing-doc.md

@@ -0,0 +1,24 @@
+<!--
+Eval-set fixture for substrate-claim-checker (B-0170).
+
+Reproduces the existence-drift pattern surfaced in PR #1252,
+where a future-domain memo referenced a docs/ markdown file
+that did not actually exist on disk. The historical eval-set
+catalogues this as instance #8 of the verify-then-claim memo's
+body table (Existence drift sub-class).
+
+The fixture references a clearly synthetic path that will
+never be created accidentally, so the fixture stays stable
+across substrate evolution.
+
+NOTE: this comment intentionally avoids backtick-quoting
+the exact fixture path. Per PR #3611 review threads
+(chatgpt-codex-connector + copilot-pull-request-reviewer),
+restating a path claim inside the HTML provenance comment
+produces a spurious second finding and lets regressions
+in body-claim detection slip past `fixtures.test.ts`.
+-->
+
+# Existence-drift catalogue (eval-set fixture)
+
+The drift instance below cites `docs/_fixture_existence_drift_target_b0170_2026_05_15.md` as the canonical source.