Host-exact REGEXP_EXTRACT compiles its pattern once

Applies to: REGEXP_EXTRACT (host-exact upcall path)

Flink's own SqlFunctionUtils.regexpExtract calls Pattern.compile on every invocation. The upcall now routes to a byte-identical reimplementation that caches the compiled Pattern per regex string instead — it's the same java.util.regex engine, so the output cannot differ, and compilation is pure, so caching it changes nothing observable.

A CPU profile put the per-call compile at ~13% of q21's total. Caching it lifted the whole query +12.5% (96 → 108 profile-loop iterations), with the compile subtree measuring zero afterward. Stock Flink pays the recompile cost on every REGEXP_EXTRACT row; we no longer do.