Tighten macOS performance budgets after eliminating the 30-second tail

[karthiknadig]

Problem

The macOS performance budgets added in #503 were calibrated while the deterministic /usr/bin/python3 probe tail was still present. They intentionally allow very large tail changes:

• server startup P95: 10,000ms absolute / 100% relative
• full refresh P95: 5,000ms absolute / 25% relative
• time to first environment P95: 10,000ms absolute / 100% relative

PR #506 eliminates that tail. Its two final validations report:

Metric	Run 1	Run 2	Existing baseline
Full refresh P95	1,435ms	1,256ms	30,320ms
Server startup P95	691ms	-	7,356ms
Time to first environment P95	745ms	-	7,433ms

With a post-fix baseline near 1-1.5 seconds, the current absolute budgets could allow a multi-second regression before blocking. That would normalize away much of #506's improvement.

Scope

After #506 merges and successful exact-base baseline artifacts exist:

• inspect repeated post-fix macOS baselines and hosted-runner variance;
• tighten macOS startup, refresh, and time-to-first P95 budgets with measured headroom;
• retain the dual absolute/relative rule so small noisy changes do not fail;
• update comparator tests and docs/QUALITY_SNAPSHOTS.md;
• leave Linux and Windows budgets unchanged unless new evidence independently justifies a change.

Acceptance criteria

• Missing or malformed metrics continue to fail closed.
• Synthetic regressions beyond each new macOS limit fail tested comparator cases.
• Repeated unchanged-head macOS runs remain green.
• A recurrence of a 15-second interpreter probe tail is unambiguously blocked.
• Full lint, CI, coverage, performance, and CodeQL checks pass.

Depends on #506.