test(pipeline): cover changed-file and empty-commit Branch HEAD refresh

[TheDarkniteFalls]

Adds the A→B→C regression coverage requested in #1213 for the already-fixed Branch HEAD behavior. The existing temporary-Git test already covers an empty commit and refreshing HEAD between pipeline construction and run start; this extends it with assertions at initial commit A and after a tracked source change at B, before the empty commit C. Each re-index checks the current SHA and rejects the preceding stale SHA.

The existing forced-full and subsequent no-op route assertions remain. This changes only tests/test_pipeline.c; no production behavior or mid-run snapshot-consistency contract changes.

Validation on macOS arm64:

• Full scripts/test.sh: 8,190 passed, 0 failed, 10 platform skips across 144 sanitizer suites; final production-binary regression checks passed.
• scripts/lint.sh --ci: passed with Homebrew LLVM 22.1.8 and cppcheck 2.21.0. The prescribed lint-tidy-diff gate reports no eligible production-source changes for this test-only patch. Local tool versions differ from CI pins.
• Changed-file clang-format and git diff --check: passed.
• Full security target: passed, including 32/32 MCP robustness cases; the Linux strace check is skipped on macOS.
• Independent read-only review: no remaining findings.

Written with OpenAI Codex assistance under human accountability. The commit carries a DCO sign-off.

Fixes #1213.

[github-actions]

Thanks for opening this — it has been seen, and it is queued.

This note is automated, but it is not a brush-off: it exists so you know where your PR stands instead of having to guess from silence.

Current review status: working through a backlog. 0.9.1-rc.1 is out, so the release freeze that held reviews is over — but it left a large queue of open pull requests behind it, and we are reading through them oldest-first. The background is in discussion #1144.

What that means for this PR, concretely:

• It will not be closed for inactivity. No stale bot touches pull requests here.
• It may still sit a while before a human reads it. That is on us, not on you.
• Older PRs are read first, so a recent one is not being skipped — it is behind a queue.

Things that will genuinely speed it up whenever review does happen:

• Keep it rebased on main — the tree is moving quickly right now, and a conflicting branch cannot be reviewed as the diff you intended.
• Get CI green, or say which failures you believe are pre-existing.
• Keep the change to one claim. Bundled features and refactors get split before they get merged, which costs you a round trip.
• Every commit needs a sign-off (git commit -s) — CI enforces DCO.

If this fixes a bug, a reproduction we can run is worth more than a description of the symptom.

Thanks for contributing, and sorry in advance for the wait.

[DeusData]

Merged. Thank you, @TheDarkniteFalls! This is exactly the A→B→C coverage #1213 asked for. Each re-index checks the current Branch HEAD SHA and rejects the stale one, at the initial commit, after a tracked source change, and across the empty commit. So a future regression in the HEAD refresh can't slip past quietly. Keeping it tests-only, with the existing forced-full and no-op route assertions intact, made it an easy yes. It went in on a fully green matrix, and nothing merged since touches tests/test_pipeline.c.