fix(physics): #146 — re-base building collision per landblock apply (lost after portal-in)

Static building/house-wall collision worked on a fresh login to a town but was
lost after logging in elsewhere and PORTALING in: the foot-sphere clipped
straight through walls while server-spawned doors (own collision) still blocked.

Root (capture-confirmed, ACDREAM_PROBE_BUILDING bldOrigin): the building shell
BSP's WorldTransform is computed from the streaming-relative origin
(origin = (lb − _liveCenter)·192) AT CACHE TIME, and CacheBuilding is idempotent
(first-wins per cell). A teleport recenters _liveCenter, but the idempotent guard
never re-bases the cached transform — so the shell sat at a stale world offset
(login Arwic → portal Holtburg: bldOrigin=(-5488,2149), ~5.5 km from the player
at (83,24)), and FindBuildingCollisions never penetrated → result=OK everywhere →
no block. Terrain doesn't suffer this because AddLandblock overwrites its
WorldOffset on every apply.

Fix: clear a landblock's cached buildings at the start of each ApplyLoadedTerrain
(PhysicsDataCache.RemoveBuildingsForLandblock), then let the existing loop
re-populate fresh with the CURRENT origin — the per-apply re-base terrain already
gets, while keeping CacheBuilding's per-cell first-wins within each fresh pass
(retail CSortCell::add_building). Also adds bldOrigin to the [bldg-channel] probe.

Verified on the exact repro (Arwic login → Holtburg portal → walk into wall):
bldOrigin now (107.5,36.0) at the wall (was -5488,2149); channel returns
Collided → Slid (block + wall-slide). Suites green: Core 1568(+2 skip),
App 468(+2 skip), UI 425, Net 317.

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

src/AcDream.Core/Physics/PhysicsDataCache.cs

@@ -455,6 +455,28 @@ public sealed class PhysicsDataCache
         };
     }
 
+    /// <summary>
+    /// #146 (2026-06-24): drop every cached building belonging to a landblock so
+    /// the next <see cref="CacheBuilding"/> pass re-bases their STREAMING-RELATIVE
+    /// <c>WorldTransform</c> against the CURRENT <c>_liveCenter</c>. Without this,
+    /// CacheBuilding's per-cell first-wins guard LOCKS the transform at whatever
+    /// frame it was first cached with; a teleport recenter then leaves the shell
+    /// BSP at a stale world offset (~the source↔dest landblock distance), so the
+    /// foot-sphere walks through where the wall visually is and collision is lost
+    /// (login at Arwic → portal to Holtburg → house walls clip; bldOrigin probed
+    /// ~5.5 km off). Terrain avoids this because <c>AddLandblock</c> overwrites its
+    /// WorldOffset every apply; buildings get the equivalent per-apply re-base via
+    /// the caller clearing-then-repopulating (which keeps first-wins within each
+    /// fresh pass — retail <c>CSortCell::add_building</c> semantics preserved).
+    /// </summary>
+    public void RemoveBuildingsForLandblock(uint landblockId)
+    {
+        uint prefix = landblockId & 0xFFFF0000u;
+        foreach (var key in _buildings.Keys)
+            if ((key & 0xFFFF0000u) == prefix)
+                _buildings.TryRemove(key, out _);
+    }
+
     public BuildingPhysics? GetBuilding(uint landcellId)
         => _buildings.TryGetValue(landcellId, out var b) ? b : null;