fix(core): Build the user regex engine lazily on reset

resetUserRegexEngine rebuilt the default engine eagerly, while every
other reset path in this module builds lazily on next use. Shutdown
paid for a second, immediately-unused engine instance every time.

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

packages/workflow/src/safe-regex.ts

@@ -212,7 +212,7 @@ export function setUserRegexEngine(regexEngine: RegexEngine): void {
 }
 
 export function resetUserRegexEngine(): void {
-	userEngine = createDefaultEngine();
+	userEngine = undefined;
 }
 
 function getUserEngine(): RegexEngine {