Harden cache runtime writes

[mehul0810]

Summary

• Writes page-cache body and metadata files through same-directory temporary files and atomic renames.
• Writes the cache body before metadata so fresh metadata never points at a missing or partial body file.
• Restricts page-cache persistence to cacheable HTML responses: HTTP 200, no Location, no Set-Cookie, and HTML content/body checks.
• Samples high-traffic cache counters (hits, stale_hits, bypasses, lock_waits) so hot cache traffic no longer persists perform_cache_stats on every request.
• Uses one uninstall option list for single-site and multisite cleanup, including cache/runtime options.

Validation

• php -l src/Modules/Cache/PageCache.php && php -l src/Modules/Cache/StatsStore.php && php -l uninstall.php
• git diff --check
• Studio reflection smoke: atomic body/meta writes created sample.html and sample.meta.json; HTML response accepted; JSON and redirect responses rejected.
• Studio stats smoke: sample rate 1 persisted a hit; large sample rate skipped repeated hit increments without creating perform_cache_stats.

Notes

• composer check-cs -- src/Modules/Cache/PageCache.php src/Modules/Cache/StatsStore.php uninstall.php is still blocked by pre-existing whole-file PHPCS violations in these files, mostly the repo's conflicting short-array rules and legacy formatting. I did not mass-format unrelated release code in this PR.

Closes #82.
Closes #83.
Closes #84.
Closes #85.

[github-actions]

Test on Playground
Test this pull request on the Playground