feat(physics): #101 — add IsPhantomGfxObjSource predicate

Retail's CPartArray::InitParts emits collision shapes only from
Setup-level CylSpheres/Spheres or per-Part PhysicsBSP — never
from visual mesh AABBs. The predicate captures the retail rule:
a stab whose source is a GfxObj (high byte 0x01) with no cached
GfxObjPhysics is phantom (no collision). Wired into GameWindow's
mesh-aabb-fallback synthesis in the next commit.

Refs docs/research/2026-05-25-a6-stairs-cyl-retail-investigation.md.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/AcDream.Core/Physics/PhysicsDataCache.cs

@@ -359,6 +359,25 @@ public sealed class PhysicsDataCache
     }
 
     public GfxObjPhysics? GetGfxObj(uint id) => _gfxObj.TryGetValue(id, out var p) ? p : null;
+
+    /// <summary>
+    /// Issue #101 (2026-05-25): retail-faithful phantom check for
+    /// GfxObj-only entity sources. Returns true when the entity's
+    /// <c>SourceGfxObjOrSetupId</c> is a GfxObj (high byte
+    /// <c>0x01</c>) AND has no cached <see cref="GfxObjPhysics"/> —
+    /// meaning the underlying GfxObj had <c>HasPhysics=False</c> or
+    /// a null <c>PhysicsBSP.Root</c>, so <see cref="CacheGfxObj"/>
+    /// short-circuited at the early-return on line 45/46. Retail's
+    /// <c>CPartArray::InitParts</c> emits NO collision shapes for
+    /// these — acdream's <c>mesh-aabb-fallback</c> synthesis at
+    /// <c>GameWindow.cs:6116</c> must do the same.
+    /// </summary>
+    public bool IsPhantomGfxObjSource(uint sourceId)
+    {
+        if ((sourceId & 0xFF000000u) != 0x01000000u) return false;
+        return GetGfxObj(sourceId)?.BSP?.Root is null;
+    }
+
     public SetupPhysics? GetSetup(uint id) => _setup.TryGetValue(id, out var p) ? p : null;
     public CellPhysics? GetCellStruct(uint id) => _cellStruct.TryGetValue(id, out var p) ? p : null;
     public int GfxObjCount => _gfxObj.Count;