stmtCacheSize is immutable after connection open, so checking it
before the lock avoids mutex overhead when cache is not enabled.

When stmtCacheSize <= 0, stmtCacheCount >= stmtCacheSize is always
true, so the explicit check is unnecessary.

Finalize all cached statements even if one fails. Leaving a
finalized statement in the cache map would be a use-after-finalize
bug per SQLite documentation.

prepareWithCache now delegates to prepare and sets cacheKey
afterward, removing the useCache boolean parameter.

This avoids an unnecessary reset when the cache is full, guarantees
a statement cannot enter the cache without being reset/cleared, and
fixes a leak where sqlite3_finalize was not called when reset failed.

Clarify that each connection in the sql.DB pool maintains its own
independent statement cache.

[codex] add opt-in statement cache