DR-042 Phase C (legibility, part 2): walls block enemies (C5) — restore the fortress fantasy

Player-built structures now physically block enemies (husks walked straight through walls before). Dedicated
"Structure" physics layer (slot 9) so the player passes its own walls while enemies are stopped:

- New WorldCollisionConfig.StructureMask, baked from the "Structure" layer in WorldCollisionAuthoring (mirrors
  EnvironmentMask). EnemyAISystem ORs it into the movement sweep filter (CollidesWith = envMask | structMask) —
  no new system, same 1-2 SphereCasts per enemy.
- Wall/Turret/Pylon prefabs get a cell-sized BoxCollider on the Structure layer (Wall's existing one relayered).
- Physics matrix: Default x Structure unchecked, so the kinematic player CC (Default) passes its own walls while
  the enemy's explicit cast still hits them. Despawn frees collision for free (collider dies with the entity).

Play-verified baked filters: StructMask=512; structure colliders BelongsTo=512, CollidesWith=0xFFFFFFFE
(includes Environment for the enemy cast, EXCLUDES bit 0 so the player passes). 389/389 EditMode, no exceptions.
Server-only/static colliders -> deterministic, no client divergence. SaveData stays v5.

Phase C complete (C5-C7). A visual fun-gate (husk stops at wall, player walks through) is the operator's eyes.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Assets/_Project/Scripts/Simulation/World/WorldCollisionComponents.cs

@@ -11,7 +11,14 @@ namespace ProjectM.Simulation
     /// </summary>
     public struct WorldCollisionConfig : IComponentData
     {
+        /// <summary>BelongsTo bitmask of the Environment physics layer (<c>1u &lt;&lt; layerIndex</c>).</summary>
         /// <summary>BelongsTo bitmask of the Environment physics layer (<c>1u &lt;&lt; layerIndex</c>).</summary>
         public uint EnvironmentMask;
+
+        /// <summary>DR-042 C5: BelongsTo bitmask of the "Structure" physics layer (player-built Wall/Turret/Pylon
+        /// colliders). OR'd into the enemy-movement sweep filter so husks collide-and-slide against walls. 0 if the
+        /// layer is absent (feature inert). The layer matrix excludes Structure&times;the player layer so the player CC
+        /// passes through its own walls.</summary>
+        public uint StructureMask;
     }
 }