docs: correct stale UCG CellGraph comments — the graph is active, not inert

The "consumed by nobody (zero behavior change)" / "INERT in Stage 1 (no writer)"
comments predate the UCG becoming load-bearing. Verified against the call sites:
CellGraph is populated unconditionally in CacheCellStruct (before the idempotency
+ null-BSP guards, so BSP-less cells are included) and consumed for the player
render/lighting root (CurrCell, written at the PhysicsEngine.UpdatePlayerCurrCell
player chokepoint; read by GameWindow:7502/7717), the universal id->cell resolver
(GetVisible), the 3rd-person camera cell (FindVisibleChildCell), and the
block-local terrain origin (TryGetTerrainOrigin, read by CellTransit:484/736).
Comments only — no behavior change. Core suite 1445 passed / 2 skipped.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/AcDream.Core/Physics/PhysicsDataCache.cs

@@ -26,8 +26,16 @@ public sealed class PhysicsDataCache
     private readonly ConcurrentDictionary<uint, BuildingPhysics> _buildings = new();
 
     /// <summary>
-    /// UCG Stage 1: the unified cell graph, built alongside the legacy cell caches.
-    /// Consumed by nobody this stage (zero behavior change).
+    /// The unified cell graph (UCG): the active id-&gt;cell resolver and registry.
+    /// Populated unconditionally in <see cref="CacheCellStruct"/> — BEFORE the
+    /// idempotency + null-BSP guards, so BSP-less cells are registered too — and
+    /// consumed across the engine: the player render/lighting root
+    /// (<c>CellGraph.CurrCell</c>, written at the player chokepoint
+    /// <c>PhysicsEngine.UpdatePlayerCurrCell</c> and read by the renderer), the
+    /// universal id-&gt;cell lookup (<c>GetVisible</c>), the 3rd-person camera cell
+    /// (<c>FindVisibleChildCell</c>), and the block-local terrain origin
+    /// (<c>TryGetTerrainOrigin</c>, read by <c>CellTransit</c>'s pick + transit
+    /// paths). No longer inert.
     /// </summary>
     public UcgCellGraph CellGraph { get; } = new();