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();
 

src/AcDream.Core/World/Cells/CellGraph.cs

@@ -6,17 +6,26 @@ using AcDream.Core.Physics;   // TerrainSurface
 namespace AcDream.Core.World.Cells;
 
 /// <summary>
-/// The unified cell graph: the authoritative id-&gt;cell resolver and registry.
-/// Built alongside the legacy render/physics cell systems in Stage 1 and consumed
-/// by nobody (zero behavior change). Retail anchor: CObjCell::GetVisible (pseudo_c:308209).
-/// Worker-thread populated; reads are concurrency-safe.
+/// The unified cell graph: the active, authoritative id-&gt;cell resolver and registry.
+/// Populated unconditionally from
+/// <see cref="AcDream.Core.Physics.PhysicsDataCache.CacheCellStruct"/> (before its
+/// idempotency + null-BSP guards, so BSP-less cells are included) and consumed across
+/// the engine: <see cref="GetVisible"/> resolves any cell id, <see cref="CurrCell"/> is
+/// the player render/lighting root, <see cref="FindVisibleChildCell"/> resolves the
+/// 3rd-person camera cell, and <see cref="TryGetTerrainOrigin"/> supplies the block-local
+/// terrain origin for the LandDefs lcoord math. Retail anchor: CObjCell::GetVisible
+/// (pseudo_c:308209). Worker-thread populated; reads are concurrency-safe.
 /// </summary>
 public sealed class CellGraph
 {
     private readonly ConcurrentDictionary<uint, EnvCell> _envCells = new();
     private readonly ConcurrentDictionary<uint, (TerrainSurface Terrain, Vector3 Origin)> _terrain = new();
 
-    /// <summary>Player's current cell. Defined for Stage 2; INERT in Stage 1 (no writer).</summary>
+    /// <summary>The player's current cell — the render/lighting root. Written ONLY at the
+    /// player chokepoint <see cref="AcDream.Core.Physics.PhysicsEngine.UpdatePlayerCurrCell"/>
+    /// (NPCs never touch it — a per-entity writer was the cottage-doorway "blue-hole"
+    /// cause); read by the renderer for the player root (GameWindow). Left unchanged when
+    /// the id isn't yet resolvable in the graph (stale beats null).</summary>
     public ObjCell? CurrCell { get; internal set; }
 
     public bool Contains(uint envCellId) => _envCells.ContainsKey(envCellId);