Release v0.2.10 — discipline-contract sprint

Closes the visibility gap on the per-coroutine isolation contract raised
in this week's public technical review:
- RequestContext::once(\$key, \$fn) helper (Laravel 11 once() analog)
- Worker-recycle access log line
- Opt-in IniIsolationMiddleware
- Coroutine safety matrix + Store semantics docs
- OPcache production tuning notes
- Handler stack reset in superglobals mode

169 unit tests, PHPStan green. See CHANGELOG.md and CRITIC.md.