Fix cache compatibility and reduce mutable kernel hashing

[brianvarskonst]

Warm mutable requests no longer hash each consumed configuration file twice. Content validation still detects same-mtime replacements and imported-resource changes; complete elimination of mutable-content reads requires the documented immutable build-ID policy.

Existing installations with group-writable compiled-cache directories boot through a fresh private cache generation. Prior PHP dumps and directory permissions are preserved, and explicit publicly served cache paths remain rejected. Prepare patch 1.1.2.

Validation: composer qa passed (70 tests/337 assertions, CS/PHPStan; seven existing PHPUnit notices). New tests reproduce the original failures and exercise successive fresh PHP processes. Runtime container/lint checks passed. A fixture benchmark with 8 MiB configuration and 2 MiB lock (20 fresh requests per state) measured median 15.52 to 10.21 ms and configuration reads 2 to 1. This is fixture performance evidence, not a production latency claim.