docs(engine-ambient): explain why _components GetOrAdd factory is contention-safe

Reviewer noted the factory may be invoked more than once under contention.
Document the analysis inline so a future reader doesn't have to redo it:
the discarded instance's mutations land on private fields of a soon-unreachable
object, and the only shared sentinel (_noopViewMaterial) is read-only.

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

SVSim.BattleEngine/Shim/UnityEngine/UnityShim.cs

@@ -225,6 +225,10 @@ namespace UnityEngine
                 var fresh = new System.Collections.Concurrent.ConcurrentDictionary<Type, object>();
                 map = System.Threading.Interlocked.CompareExchange(ref _components, fresh, null) ?? fresh;
             }
+            // GetOrAdd may invoke the factory more than once under contention; only one result wins.
+            // Safe here: the discarded instance has its _go set to `this` (private write to a soon-
+            // unreachable object) and WireComponentFields only assigns to the new tree's own private
+            // fields. The shared _noopViewMaterial sentinel below is read-only. No global state leaks.
             return map.GetOrAdd(t, ty =>
             {
                 object inst;