fix(physics): #101 — suppress mesh-aabb-fallback for phantom GfxObj stabs

The 10 stair-step cyls (entities 0x40B5008A..0x40B50095 in Holtburg
cells 0xA9B40159/A) are synthesized by the mesh-aabb-fallback path
from the visual mesh AABB of GfxObj 0x0100081A — which has
HasPhysics=False and no PhysicsBSP. Retail's CPartArray::InitParts
emits no collision in this case; acdream now matches that by
consulting PhysicsDataCache.IsPhantomGfxObjSource (added in the
previous commit) and skipping synthesis when the predicate fires.

The actual staircase collision is on entity 0x40B50089 (GfxObj
0x01000C16, hasPhys=True, BSP radius 2.645m) — same staircase BSP
that retail uses. After this fix, only that BSP fires; the
phantoms are gone.

Visual verification pending (next step in plan); the BSP dump
from ACDREAM_DUMP_GFXOBJS=0x01000C16 will confirm whether
0x01000C16 has walkable inclined polys for the climb to actually
land. If not, a follow-up issue is needed; the cyl phantom is
closed either way.

Also updates PhysicsDataCache.cs XML doc line reference from
6116 to 6127 (drifted by the 11-line isPhantomGfxObj block
inserted above the guarded if).

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

@@ -370,7 +370,7 @@ public sealed class PhysicsDataCache
     /// 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.
+    /// <c>GameWindow.cs:6127</c> must do the same.
     /// </summary>
     public bool IsPhantomGfxObjSource(uint sourceId)
     {