refactor(M03): extract terrain quality mesh cache

RENDER.md

@@ -979,6 +979,18 @@ $exe = Join-Path $env:TEMP 'godot-4.6.1-openwc\Godot_v4.6.1-stable_win64.exe'
 - PackedScene loading is currently synchronous on create/replace and is a documented
   prototype limitation, not an accepted network-scale frame-critical path.
 
+## 2026-07-16 Terrain Quality Mesh Cache Extraction
+
+- `TerrainQualityMeshCache` now owns full-quality terrain revisit Mesh references,
+  tile-key LRU order and source admission outside `StreamingWorldLoader`.
+- The historical rules are unchanged: successful restore touches newest; store
+  trims oldest to `terrain_quality_mesh_cache_limit`; zero/negative capacity keeps
+  no entries; control-splat/splat sources are never retained in this revisit cache.
+- The loader keeps three narrow adapters: restore into tile state, store after a
+  full-quality result and clear during existing world reset/shutdown.
+- ADT parsing, quality tasks/results, tile state, cache format versions, material/
+  Node/RID finalization, budgets and visible terrain behavior remain loader-owned.
+
 ## Practical Rule For Future Work
 
 If something improves quality but creates visible hitch, it is not done. Move it to bake/cache/background work, split finalization over frames, or prewarm it before the player can see it.