fix(physics): Slice 1 — delete render-mesh-AABB synthetic collision; DAT-only shape authority

Retail oracle: CPartArray::InitParts@0x00517F40, CGfxObj::Serialize@0x00534970 (physics_bsp
gated on serialized-flags bit-0), CPhysicsPart::find_obj_collisions@0x0050D8D0 (returns OK /
passable when physics_bsp==null). Render-mesh bounds never enter collision.

Changes:
- GameWindow.cs: delete the ~200-line VISUAL mesh-bounds collision block (the
  isPhantomSetup / isPhantomGfxObj locals + the if-block computing worldMin/worldMax
  AABB + the ShadowObjects.Register call that capped and registered the synthetic
  cylinder). Also removes dead counter variables scHaveBounds/scRegistered/scNoBounds/
  scTooThin; trims the ProbeBuildingEnabled summary line accordingly.
- PhysicsDataCache.cs: delete IsPhantomGfxObjSource (the predicate that only existed
  to fence the mesh-AABB synthesis; the "phantom" concept is now the default — no DAT
  shape means no registration, verbatim with retail).
- PhysicsDataCachePhantomSourceTests.cs: deleted (tested the removed method).
- ShadowShapeBuilderShapeSourceTests.cs: new guard test — a Setup with parts but
  hasPhysicsBsp=false and no CylSpheres/Spheres yields an empty shape list, locking
  the DAT-only rule in the builder.
- retail-divergence-register.md: AP-2 row deleted (divergence retired).

Objects with no DAT physics shape (no CylSpheres, no Spheres, no part with a
PhysicsBSP) now register no collision shape and are passable, verbatim with retail.
Objects with real DAT shapes (BSP parts, CylSpheres) are unaffected.

dotnet build green, 22/22 tests passing (ShadowShapeBuilderShapeSourceTests +
CellarUpTrajectoryReplay + CornerFlood + Issue147ArwicBuildings replay harnesses).

Visual gate pending: walk Holtburg + open world; objects that become passable must
match retail (DAT has no physics shape — trees with real CylSpheres still solid).

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

docs/architecture/retail-divergence-register.md

@@ -104,7 +104,7 @@ accepted-divergence entries (#96, #49, #50).
 | # | Divergence | Where (file:line) | Why it is safe / justified | Risk if assumption breaks | Retail oracle |
 |---|---|---|---|---|---|
 | AP-1 | Snap-path Z settle: validated claims ground on their own walkable polys, but floor-less claims (thresholds, stair lips) fall through to a legacy nearest-in-Z scan over every CellSurface in the landblock; retail settles via `CheckPositionInternal` → `find_valid_position` | `src/AcDream.Core/Physics/PhysicsEngine.cs:614` | `find_valid_position` unported; the **#111** fix narrowed the legacy pick's blast radius (validated claims bypass it) rather than replacing it | A threshold/stair-lip snap can still pick a neighbouring cell's same-height floor by iteration order — wrong cell or Z at login/teleport arrival (the #111 clobber class) | `SetPositionInternal` :283426 → find_valid_position |
-| AP-2 | Visual-AABB fallback collision shape for Setups with no retail physics data; retail emits NO shapes (phantom). **#101** fixed the GfxObj-only class; the Setup-without-shapes fallback remains | `src/AcDream.Core/Physics/PhysicsDataCache.cs:96` | Lets the player collide with decorative meshes shipping no CylSphere/part-BSP instead of walking through furniture-like props | Retail-phantom entities block movement (the **#100/#101** family), and the synthetic box gives non-retail push-out normals when it collides | `CPartArray::InitParts` (cited at PhysicsDataCache.cs:386-389) |
+
 | AP-3 | Step-down chain triggered only when contact is invalid OR steeper than walkable; retail's `transitional_insert` OK-path ALWAYS runs it | `src/AcDream.Core/Physics/TransitionTypes.cs:1197` | Conditional preserves the observed-to-matter cases (edge departure, steep cliff-slide) without running the chain every step (per pc:273191 agent reports) | Steps where retail runs step-down despite a valid walkable contact (bump maintenance, edge-slide arming) are skipped — float-off or missed edge slides in untested geometry | `transitional_insert` OK-path pc:273191 |
 | AP-4 | CliffSlide check moved BEFORE retail's Branch-1 (`!OnWalkable` → restore+OK) gate, compensating our L.2.3i FloorZ OnWalkable bookkeeping | `src/AcDream.Core/Physics/TransitionTypes.cs:1316` | Retail's order with our incomplete OnWalkable stops the player dead every frame on steep slopes ("stay on the roof"); reorder restores downhill drift | CliffSlide fires in states where retail's Branch 1 would restore-and-OK — body slides where retail holds, e.g. contact-plane-bearing steep geometry near edges | retail EdgeSlide dispatch order (transitional_insert step-down failure) |
 | AP-5 | Step-down skips Placement validation for the contact-maintenance call (`runPlacement=false`); ACE/retail run it unconditionally (kept for DoStepUp) | `src/AcDream.Core/Physics/TransitionTypes.cs:3393` | Residual wall-slide artifacts made Placement misfire, leaving players stuck near walls; the skip was the targeted L.2.3h fix | Step-down can settle into positions Placement would reject — slight wall embedding, or accepting a step-down through overlap geometry retail catches | `CTransition::step_down` pc:272952; ACE Transition.cs:731-741 |