ci: schedule Rust workspace tests on macOS or Linux self-hosted runners

.github/actionlint.yaml

@@ -0,0 +1,9 @@
+# Custom labels carried by this repo's self-hosted runners. Without this,
+# actionlint flags `runs-on: [self-hosted, rust-ci]` as an unknown label
+# because it can't resolve it against the GitHub-hosted runner names.
+#
+# `rust-ci` must be applied to every runner allowed to take the Rust workspace
+# test job (Settings -> Actions -> Runners -> <runner> -> Labels).
+self-hosted-runner:
+  labels:
+    - rust-ci

.github/workflows/tests-rs-wallet.yml

@@ -19,11 +19,12 @@
 # through the fast path that the full path would have rejected.
 #
 # Coverage upload is intentionally omitted (codecov never gates the merge —
-# `fail_ci_if_error: false`). Known trade-offs: no Ubuntu backup jobs exist
-# here (if `UBUNTU_BACKUP_ENABLED` is turned on, wallet PRs still depend on
-# the mac runner being online), and the scoped `-p` builds feature-unify
-# shared deps differently than `--workspace` builds, so the shared target/
-# carries an extra artifact flavor.
+# `fail_ci_if_error: false`). Known trade-offs: this fast path stays pinned to
+# the macOS runners (unlike the full workspace job, which schedules onto any
+# `rust-ci` self-hosted runner), so wallet PRs depend on a mac runner being
+# online; and the scoped `-p` builds feature-unify shared deps differently
+# than `--workspace` builds, so the shared target/ carries an extra artifact
+# flavor.
 on:
   workflow_call:
     inputs:
@@ -198,8 +199,7 @@ jobs:
 
       # Wallet-only PRs can only change doc examples in the wallet crates, so
       # scoped doctests are equivalent to the full job's `--workspace --doc`
-      # run for this scope. (The standalone rs-doctests job in tests.yml is
-      # only an Ubuntu backup gated on UBUNTU_BACKUP_ENABLED.)
+      # run for this scope.
       - name: Run doctests (wallet crates)
         if: ${{ inputs.doctests-changed }}
         run: |