feat(audience): add ImmutableAudience singleton + GDPR (SDK-147)

Adds the ImmutableAudience static entry point that ties DiskStore,
EventQueue, HttpTransport, ConsentStore, and Identity together into
the public SDK surface for track/identify/alias plus the GDPR
lifecycle (SetConsent, Reset, DeleteData).

Public API: Init, Track (IEvent + string overloads), Identify,
Alias, Reset, DeleteData, SetConsent, FlushAsync, Shutdown.

Behavioural guarantees:
- Concurrency: all public methods are thread-safe. Track is the
  allocation-light hot path - it enqueues a dictionary and returns;
  serialisation runs on the drain thread.
- Consent state machine: None drops everything; Anonymous
  discards userId from track messages and drops identify/alias;
  Full sends everything. Downgrades rewrite the on-disk queue.
- Never-throw: public methods catch and route every exception to
  the OnError callback, including background drain failures and
  shutdown flush errors.
- Shutdown hygiene: final synchronous drain, drain-thread join
  with timeout, HttpTransport disposal, idempotent (safe to call
  from application exit + Unity quit).
- Persistence round-trip: identity, consent, and queued events
  all survive process restart via atomic write-temp-then-move.

Code changes:
- ImmutableAudience.cs: 549-line rewrite of the stub (Init wiring,
  public API surface, consent state machine, backend sync loop).
- AudienceConfig.cs: adds OnError, PersistentDataPath,
  PackageVersion, ShutdownFlushTimeoutMs, HttpHandler (test seam).
- ConsentLevel.cs: ConsentLevelExtensions.ToWireString for the
  tracking-consent backend audit payload.
- Transport/EventQueue.cs: swaps ConcurrentQueue<string> for
  ConcurrentQueue<Dictionary<string,object>> so Json.Serialize
  runs on the drain thread; adds _drainLock to serialise drain
  against PurgeAll / ApplyAnonymousDowngrade so a TryDequeue'd
  event cannot hit disk after consent revocation wiped it;
  EnqueueChecked closes the same window for new Track calls that
  race SetConsent(None). See _drainLock comment for full race.
- Transport/DiskStore.cs: DeleteAll and ApplyAnonymousDowngrade
  for the queued-event side of consent revocation. The latter
  strips userId from queued track messages via JsonReader ->
  Json.Serialize round-trip (numeric-precision caveat documented
  in-file - exact for realistic Purchase.Value amounts).

Tests:
- ImmutableAudienceTests.cs: 737 lines covering Init/Shutdown
  lifecycle, the Track/Identify/Alias/Reset/DeleteData/FlushAsync
  public surface, the consent state machine transitions, the
  never-throw contract, and the privacy invariants.
- ConsentSyncTests.cs: backend consent audit round-trip.
- DeleteDataTests.cs: queue-purge + backend-call + Identity cache
  clearing interaction.
- Transport/DiskStoreTests.cs, Transport/EventQueueTests.cs:
  coverage for the new GDPR ops.

The preceding four peels (remove duplicate IdentityTests, mirror
Runtime layout onto Tests, SSOT constants + scaffolding, ConsentStore)
were carved out of this commit to keep the singleton diff focused on
the parts that only make sense together.

Author: ImmutableJeffrey

src/Packages/Audience/Runtime/AudienceConfig.cs

@@ -1,3 +1,5 @@
+using System;
+
 namespace Immutable.Audience
 {
     // Configuration passed to ImmutableAudience.Init.
@@ -20,5 +22,20 @@ public class AudienceConfig
 
         // Flush as soon as this many events are queued.
         public int FlushSize { get; set; } = Constants.DefaultFlushSize;
+
+        // Optional error callback.
+        public Action<AudienceError> OnError { get; set; }
+
+        // Directory the SDK uses for identity, consent, and queued events.
+        public string PersistentDataPath { get; set; }
+
+        // Library version sent on every message.
+        public string PackageVersion { get; set; } = Constants.LibraryVersion;
+
+        // Maximum time Shutdown waits for the final flush.
+        public int ShutdownFlushTimeoutMs { get; set; } = 2_000;
+
+        // Test seam for HttpTransport; not part of the public API.
+        internal System.Net.Http.HttpMessageHandler HttpHandler { get; set; }
     }
 }

src/Packages/Audience/Runtime/ConsentLevel.cs

@@ -10,4 +10,18 @@ public enum ConsentLevel
         // Full tracking, including identity.
         Full
     }
+
+    internal static class ConsentLevelExtensions
+    {
+        // Throws on unknown casts rather than emitting null: a null value
+        // would poison the backend consent log.
+        internal static string ToWireString(this ConsentLevel level) => level switch
+        {
+            ConsentLevel.None => "none",
+            ConsentLevel.Anonymous => "anonymous",
+            ConsentLevel.Full => "full",
+            _ => throw new System.ArgumentOutOfRangeException(
+                nameof(level), level, "Unhandled ConsentLevel"),
+        };
+    }
 }