[Bug] Perf bench-count drift guard doesn't run when BenchCatalog.All changes, so /perf breaks after merge #1373

What happened?

tests/stress_perf/ci/PerfLib.Tests.ps1 has a drift guard: it parses BenchCatalog.All in tests/perf_bench/PerfBench.ControlModel/Benches/AllBenches.cs and asserts that the bench count equals the hand-maintained $script:MicroExpectedBenchCount in tests/stress_perf/ci/PerfLib.ps1 (currently 17). It's meant to stop the two from silently diverging.

The guard doesn't run when it matters. .github/workflows/perf-lib-tests.yml only triggers on changes under tests/stress_perf/ci/**. A PR that adds or removes a bench only touches tests/perf_bench/..., so the guard never runs, and the PR goes green with a mismatched count.

The mismatch only shows up after merge, the next time someone runs /perf. perf-compare.yml runs PerfLib.Tests.ps1 as its first step, the drift assertion fails, and the benchmark run aborts before it measures anything.

Expected: a PR that changes the number of entries in BenchCatalog.All without updating MicroExpectedBenchCount should fail CI on that PR.

Found while reviewing #1363, which adds six ItemsHost benches (IH1–IH6) to BenchCatalog.All (17 → 23). Its CI is fully green even though the count wasn't updated.

Suggested fix:

• Add the bench catalog to the perf-lib-tests.yml path filters, for example tests/perf_bench/PerfBench.ControlModel/**, for both push and pull_request. The guard is pure source parsing on a Linux runner, so this costs seconds.
• Optionally, have the drift assertion's failure message say exactly which literals to update. Today that's PerfLib.ps1:100, PerfLib.Tests.ps1:1137 and :1163; Run-PerfBenchmark.ps1 also mentions "17 benches" in its comments.
• Optionally, make the job a required check, or fold it into an always-run job, so a path-filtered skip can't hide it.

Steps to reproduce

1. On main, add one new SomeBench(), entry to BenchCatalog.All in tests/perf_bench/PerfBench.ControlModel/Benches/AllBenches.cs, without touching tests/stress_perf/ci/.
2. Open a PR. Perf lib tests doesn't run, and CI is green.
3. Run pwsh ./tests/stress_perf/ci/PerfLib.Tests.ps1 locally, or run /perf after merge. The drift guard: BenchCatalog.All declares 17 benches assertion fails.

Reactor version / commit

main @ 60b3a10. Also reproduced in the #1363 head, 92ea6ce74.

Platform

Both

.NET SDK version

10.0.402 (not SDK-dependent; this is a CI path-filter gap)

Windows version

N/A (the workflow runs on ubuntu-latest)

Windows App SDK version

N/A

Logs / stack trace

tests/stress_perf/ci/PerfLib.Tests.ps1:1163
  Assert-Equal 17 $catalogCount  'drift guard: BenchCatalog.All declares 17 benches (M1-M13 + OAlloc/OUpdate/C207 + M14)'
.github/workflows/perf-lib-tests.yml
  on.pull_request.paths: ['tests/stress_perf/ci/**', '.github/workflows/perf-lib-tests.yml']   # AllBenches.cs not included

Confirmation

• I have searched existing issues and this isn't a duplicate.
• This bug reproduces against the current main branch.