Perf: add benchmark regression gates and historical evidence

[chrisbbreuer]

Parent: #44

Goal

Keep performance work measurable and prevent wins from silently regressing without turning ordinary CI into a noisy hardware contest.

Acceptance criteria

• Store versioned raw samples and rendered reports with exact commit/environment/power metadata.
• Add deterministic standalone harness/unit checks to ordinary CI.
• Add a manually triggered or controlled-runner full performance workflow.
• Compare only like-for-like host/runtime configurations and retain historical baselines.
• Define a minimum timing floor and expose repeated-sample dispersion; noise-aware regression thresholds remain to be derived from history.
• Fail publication when the tracked worktree is dirty and capture metadata before generated outputs change the tree.
• Link optimization commits and resulting benchmark deltas from their workload issues.
• Keep README headline numbers generated from the latest accepted report rather than manually copied.

Evidence

• Publication guard: e70eeff
• Symmetric matrix and seven standalone structural tests: c9f68e3
• CI harness leg: c9f68e3
• Latest 616-sample report/TSV/README: 2ebda1a
• Benchmark source commit measured by that report: c9f68e3

[chrisbbreuer]

Progress in e70eeff and 6fb3b28: publication now refuses a dirty tracked worktree, captures metadata before writing tracked outputs, and retains the exact source commit plus 224 raw samples. #50 remains open for CI harness tests, controlled performance runners/history, derived regression thresholds, and generated README tables.

[chrisbbreuer]

The publication guard and dated evidence passed the final combined gate (31 success, 0 failure): https://github.com/zig-utils/zig-js/actions/runs/29280193748. Remaining CI/history/generation work stays open.

[chrisbbreuer]

Latest accepted evidence is now published:

• measured source: 42d0ec26
• 616-sample TSV + generated report: ccee8842
• README + benchmark guide: 09d543c3

The clean full matrix passed every checksum and 50 ms floor guard. zig-js wins all 36 symmetric single/steady/cold rows. README headline values are still manually summarized, so the generated-headline/workflow acceptance items remain open.

[chrisbbreuer]

Final validation for the published comparison is green on main at 889611de:

• CI run: https://github.com/zig-utils/zig-js/actions/runs/29305518043
• 32/32 runnable jobs passed; the single nightly-only entry was correctly skipped
• Includes the docs build, benchmark comparison harness, all unit shards, thread/reference/no-GIL checks, thread-fuzz profiles, TSan unit coverage, and all four TSan no-GIL corpus shards
• The two follow-up CI fixes are included: isolate property quickening mode in its unit test (e8f98e49) and honor the committed Bun lockfile for docs (889611de)

The measured report (ccee8842) and README summary (09d543c3) are therefore validated end-to-end. I am leaving #50 open only for its remaining automation work: generate/check the README summary from the committed report so future benchmark updates cannot drift.

[chrisbbreuer]

Pushed 0e22e4bb (bench: align comparison allocators (#50)). Every measured zig-js context now uses the same process-wide libc allocator, matching JSC's process-scoped cached allocator model instead of mixing the runner allocator into direct/shared rows. The generator and benchmark guide document the current method while preserving the July 13 report as immutable historical evidence.

Validation:

• ReleaseFast comparison runners built successfully
• benchmark harness contract: 7/7 passed
• exact array checksum stayed 23508000000 at one lane and 188190000000 at eight shared lanes
• 7-sample medians were neutral within noise: direct arrays ~61.10 -> ~61.30 ms; shared one lane ~63.15 -> ~63.49 ms; shared eight lanes ~157.59 -> ~156.23 ms

This is a fairness/alignment change, not a claimed speedup. The next accepted full matrix will measure the unified allocator rather than rewriting the saved report.