The ref hygiene gate dies on a mid-rename tree instead of reporting (#940)

[philcunliffe]

Deferred finding 1 from the triage of PR #933, carried into #940.

What was wrong

trackedFiles() in test/core/llp-ref-hygiene.test.js fed git ls-files -z output straight into fs.readFileSync with no presence filter. ls-files answers from the index, not the working tree, so a path that is tracked but absent from disk (mid-rebase, mid-rename) threw ENOENT at module load. The whole gate then reported nothing at all instead of reporting that the annotations are fine. Loud, not silent, and developer-tree only, which is why it was deferred rather than blocking.

The fix

Filter the listing with fs.existsSync(path.join(REPO_ROOT, f)) before reading, with a comment saying why the index and the working tree can disagree.

The regression test

test/core/llp-ref-hygiene-absent-path.test.js, one test: the ref hygiene gate reports a result on a tree missing a tracked file.

It exercises the real gate rather than a copy of its logic. It builds a git shim on PATH that delegates to the real binary except that ls-files -z appends one extra path with no file behind it, then spawns node --test test/core/llp-ref-hygiene.test.js and asserts the run reports test results, never prints ENOENT, and exits 0.

One subtlety worth flagging for review: the child's environment has NODE_TEST_CONTEXT deleted. The node test runner refuses nested runs (node:test run() is being called recursively within a test file. skipping running files.), and without clearing it the child exits 0 having loaded nothing, so every assertion passes against a gate that never ran. The first assertion (# tests appears in the output) is the guard against that failure mode coming back.

Evidence

=== WITHOUT FIX ===
not ok 1 - the ref hygiene gate reports a result on a tree missing a tracked file
# pass 0
# fail 1
=== WITH FIX ===
ok 1 - the ref hygiene gate reports a result on a tree missing a tracked file
# pass 1
# fail 0

Without the fix the failure message carries the reported symptom verbatim: Error: ENOENT: no such file or directory, open '.../src/core/absent-from-the-working-tree.js' at .../llp-ref-hygiene.test.js:336.

Full suite: npm test 4529 pass, 0 fail, 1 skipped. npm run typecheck clean.

Fixes #940

[philcunliffe]

⚠️ Duplicate fix — this PR and #948 fix the same defect

Issues #940 and #942 are follow-ups from two different PRs (#929 and #933), but both
enumerated the same deferred finding: trackedFiles() in
test/core/llp-ref-hygiene.test.js feeds git ls-files -z output into
fs.readFileSync with no presence filter. Two workers fixed it independently.

• The @ref hygiene gate dies on a mid-rename working tree instead of reporting (#942) #947 (this PR, fix/issue-942) extracts the listing into a shared
  test/helpers/tracked_files.js and uses it at both call sites.
• The ref hygiene gate dies on a mid-rename tree instead of reporting (#940) #948 (this PR, fix/issue-940) adds the existsSync filter inline, plus a
  subprocess-based regression test that shims git.

Both branch off master and both modify test/core/llp-ref-hygiene.test.js, so
whichever merges second will conflict.

This needs a human call: pick one approach (or merge one and rebase the other onto
it). neutral is holding both and will not merge either. The regression tests differ in
kind and are not redundant, so the surviving PR may be worth taking the other's test
with it.

[philcunliffe]

Closing as consolidated into #947. #948's subprocess integration test now runs in #947 alongside the shared tracked-file helper and unit coverage; it was adjusted only to accept both Node test-summary prefixes, then passed locally and on the Node 22/24 CI matrix before #947 merged.