chore(engine-ambient): harden shim + LocalLog statics for fixture parallelism

Follow-up to the multi-instancing migration. Wraps the process-shared engine
statics that aren't ambient-fronted but race between concurrent battles:

- UnityEngine.Resources._loaded: Dictionary -> ConcurrentDictionary.GetOrAdd
  (the shared prefab cache keyed by path; concurrent first-misses produced
  duplicate GameObjects + Dictionary corruption)
- UnityEngine.GameObject._components: Dictionary -> ConcurrentDictionary with
  Interlocked.CompareExchange init (Resources.Load returns SHARED prefab
  GameObjects, so two engines' Setup() can race on the same _components map
  — surfaced as "Operations that change non-concurrent collections" crashes
  during BattleManagerBase ctor's GetComponent<T>() chain)
- Wizard.LocalLog: single static lock around all mutating entry points
  (StringBuilder _lastTraceLogStringBuilder + ~12 mutable string/bool/int
  scratch fields; serializing the trace-log surface is cheap since logging
  is not the hot path)

Flips SVSim.BattleEngine.Tests assembly Parallelizable scope from Self to
Fixtures and restructures MultiInstanceEngineTests.StressN_BaselineMatches so
Setup runs INSIDE Task.Run (was previously serialized as a workaround for the
LocalLog races). The fixture is also lifted to ParallelScope.All so the
two-engines and stress tests can run alongside each other.

Suite fully green under fixture parallelism (59/0/2 across 3 consecutive runs);
SVSim.UnitTests still 1054/0/0 — true multi-instance correctness is now proved
end-to-end in tests rather than gated behind a serial workaround.

Manifest sha refresh + new patch artifact for the LocalLog edit (decomp-origin);
the two shim files are authored, so no metadata update is needed for them.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

SVSim.BattleEngine/Shim/UnityEngine/UnityRuntime.cs

@@ -54,13 +54,13 @@ namespace UnityEngine
         // dictionary that the resolution-path ctor then Instantiate()s + GetComponent()s
         // (e.g. Prefab/Game/UnityEventAgent). Return a cached no-op GameObject per path so that
         // chain yields a non-null object. Typed asset loads go through the generic Load<T> (null).
-        private static readonly System.Collections.Generic.Dictionary<string, GameObject> _loaded
-            = new System.Collections.Generic.Dictionary<string, GameObject>();
+        // ConcurrentDictionary + GetOrAdd so concurrent battle setups don't race on first-miss.
+        private static readonly System.Collections.Concurrent.ConcurrentDictionary<string, GameObject> _loaded
+            = new System.Collections.Concurrent.ConcurrentDictionary<string, GameObject>();
         public static Object Load(string path)
         {
             if (string.IsNullOrEmpty(path)) return null;
-            if (!_loaded.TryGetValue(path, out var go)) { go = new GameObject(path); _loaded[path] = go; }
-            return go;
+            return _loaded.GetOrAdd(path, static p => new GameObject(p));
         }
         public static Object Load(string path, Type t) => Load(path);
         public static T[] LoadAll<T>(string path) where T : Object => new T[0];