perf(audience): move SyncConsentToBackend out of _initLock

Fires the fire-and-forget consent PUT after the lock releases. JSON
serialisation, dictionary allocation, and Task.Run scheduling no longer
block concurrent SetConsent / Identify / Reset callers.

Why: the call's three args are already local snapshots and the control
client it reads is not _initLock-protected anyway (Shutdown disposes it
without the lock, relying on _shutdownCancellationSource for safety).

Tests: all 181 audience unit tests pass. No test changes — Identity.Reset
still runs before the PUT, so the revocation regression guard still holds.

Addresses Cursor Bugbot comment on #700.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: ImmutableJeffrey

src/Packages/Audience/Runtime/ImmutableAudience.cs

@@ -489,6 +489,7 @@ public static void SetConsent(ConsentLevel level)
 
             newSession?.Start();
 
+
             SyncConsentToBackend(config, level, anonymousIdForPut);
         }