diag(render): tripwires on every silent dat-miss path (white-walls attribution, #105)

The intermittent white-cottage-walls failure has NEVER produced a log line:
every dat-read failure on the walls-relevant paths exits silently, and the
failed result is cached for the session (mesh batches build once). Today it
reproduced on a probe-free launch with a 35-line, zero-error log — so the
prior heavy-probes-starve-the-dat-reader framing is not the whole story.

Tripwires (print ONLY on anomaly; zero cost healthy; keep until #105 closes):
- [dat-miss]  DatDatabaseWrapper.TryGet — a miss for an id whose BTree entry
  EXISTS (re-probed under the same lock); legit not-found fallbacks stay quiet.
- [tex-miss]  TextureCache.DecodeFromDats x3 — render-thread decode fell back
  to magenta (Surface / SurfaceTexture / RenderSurface miss).
- [cell-miss] GameWindow interior hydration x2 — EnvCell or Environment read
  returned null, so a cell''s WALLS are silently never registered while its
  statics still draw (the exact observed geometry signature).

Color discriminates the layer on the next occurrence: magenta = TextureCache;
see-through + [tex-skip]/[dat-miss] = mesh build; see-through + [cell-miss] =
hydration; broken with NO tripwire output = GL-side upload/residency.

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

src/AcDream.App/Rendering/Wb/DatCollectionAdapter.cs

@@ -149,6 +149,17 @@ internal sealed class DatDatabaseWrapper : IDatDatabase
                 _cache.TryAdd((typeof(T), fileId), value);
                 return true;
             }
+
+            // TEMP diagnostic (dat-race investigation 2026-06-09, strip with fix):
+            // a miss for an id whose BTree entry EXISTS is always an anomaly —
+            // either Unpack returned false or the lookup flickered transiently.
+            // Legit not-found probes (e.g. Portal→HighRes fallback) stay silent.
+            if (_db.Tree.TryGetFile(fileId, out _))
+            {
+                Console.WriteLine(
+                    $"[dat-miss] {typeof(T).Name} 0x{fileId:X8} entry EXISTS but TryGet failed " +
+                    $"(thread={Environment.CurrentManagedThreadId})");
+            }
         }
 
         return false;