perf(pcre): cache compiled regex patterns across preg_* calls

Composer's classmap generator re-issues the same PHP-derived pattern
string for every scanned file (and every token within it), relying on
PCRE's built-in compiled-pattern cache to make that free. shirabe had
no equivalent, so every Preg::* call recompiled the pattern from
scratch via regex::Regex::new(), making `composer create-project`
autoload generation ~130x slower than Composer on a fresh laravel/laravel
install (130s vs ~1s). Memoizing compiled patterns by their raw string
in compile_php_pattern closes that gap.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

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