1.1.0-alpha2: bundle 1.0.1 correctness fixes

Cherry-picks the four 1.0.1 fixes onto the 1.1.x branch so
operators upgrading 1.0.0 → 1.1.0 directly get all the
corrections without going through 1.0.1.

- request_id strict-regex validation in BeaconRequest.
- Top-level try/catch in BeaconController::receive() preserving
  the docblock 'never 5xx' contract.
- Cwv-rid Server-Timing entry skipped on shared-cacheable
  responses (prevents request-ID corruption on edge-cached
  pages).
- PERFORMANCE.md sampling-rate clarification (the multiplied
  effective rate).

No new 1.1.0 features beyond what alpha1 shipped (outlier
filtering, per-metric/adaptive sampling, deploy-event
annotations); alpha2 is purely the merged correctness fixes.

See ROADMAP.md 1.0.1 and 1.1.0 entries for full descriptions.