fix(physics): #42 skip self in FindObjCollisions — airborne XY drift

Root cause confirmed via two-run diagnostic and the named-retail decomp:
the airborne sweep was colliding with the moving entity's OWN ShadowEntry
because FindObjCollisions had no self-skip filter. Live entities (local
player, remotes) register a Cylinder in ShadowObjectRegistry on spawn
(GameWindow.cs:2545) and UpdatePosition tracks its world position each
tick, so the moving sphere's own cylinder is always at the body's
position. Without a gate, CylinderCollision sees the sphere overlapping
its own cylinder volume and slides the sphere ~1m horizontally on every
frame the path produces non-zero motion.

Why grounded mostly hides it and airborne exposes it:
- Stationary grounded → numSteps=0, TransitionalInsert never runs.
- Walking grounded → push fires but motion escapes the cyl radius and
  the deflection blends into normal motion.
- Stationary airborne (jump) → pure +Z motion; the cyl push is the
  only horizontal contribution and manifests as a clean ~1m drift.

Run-2 evidence (launch-42-r2.log) — 152 [SWEEP-OBJ] events, every one
with type=Cylinder, gfxObj=0x02000001 (humanoid setup), R=0.679,
H=1.835, at obj.Position EXACTLY matching the body's pre.Position. Run
1 had already ruled out H1 (cpN=(0,0,1) flat, no slope projection).

Retail does the same skip — CObjCell::find_obj_collisions at
named-retail acclient_2013_pseudo_c.txt:308931:

    if ((physobj->parent == 0 && physobj != arg2->object_info.object))

`arg2->object_info.object` is the OBJECTINFO::object self-pointer set
by OBJECTINFO::init at acclient_2013_pseudo_c.txt:274435. Our port
mirrors this with an EntityId-based filter:

  - ObjectInfo gains a SelfEntityId field (default 0 = no filter).
  - ResolveWithTransition gains an optional `uint movingEntityId = 0`
    parameter that sets it.
  - FindObjCollisions skips entries whose EntityId matches
    SelfEntityId when the id is non-zero.
  - PlayerMovementController gains a LocalEntityId property; GameWindow
    refreshes it per-tick from `_entitiesByServerGuid[_playerServerGuid]`.
  - GameWindow's airborne-remote ResolveWithTransition call site passes
    `movingEntityId: kv.Key` (kv.Key is the local entity id keying
    `_animatedEntities`, same id used at the spawn-time
    ShadowObjects.Register).

Default 0 keeps tests and one-shot callers (no registered ShadowEntry)
working unchanged.

Lock-the-fix unit test:
`PhysicsEngineTests.ResolveWithTransition_SelfShadowEntry_NotPushedWhenIdMatches`
registers a humanoid Cylinder at the body's exact position (matching
GameWindow's spawn pattern), then asserts that:
  - movingEntityId=0 (control)        → unfiltered XY drift > 0.5m
  - movingEntityId=registered id (fix) → XY drift ≈ 0

Diagnostic wiring (a36369d + this commit's [SWEEP-OBJ] addition) stays
in tree, env-var gated (ACDREAM_AIRBORNE_DIAG=1) so it produces no
output in normal use but lets us verify the fix on the live client and
debug future regressions.

Build: green. Tests: 355 pass, 6 fail (all pre-existing per the handoff
prompt — verified by stashing this change; the BSPStepUp C3 failure is
on the prior commit too).

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

src/AcDream.App/Input/PlayerMovementController.cs

@@ -130,6 +130,17 @@ public sealed class PlayerMovementController
     public Vector3 Position => _body.Position;
     public uint CellId { get; private set; }
 
+    /// <summary>
+    /// Local-player entity id used to skip self-collision in the
+    /// airborne sweep. GameWindow updates this whenever the local
+    /// `+Acdream` entity (re)spawns. Default 0 = no filter (matches
+    /// retail's CObjCell::find_obj_collisions self-skip when the
+    /// caller's OBJECTINFO::object pointer is null). Without this the
+    /// sweep collides with its own ShadowEntry registered at
+    /// GameWindow.cs:2545 — see #42.
+    /// </summary>
+    public uint LocalEntityId { get; set; }
+
     public bool IsAirborne => !_body.OnWalkable;
 
     /// <summary>
@@ -558,7 +569,12 @@ public sealed class PlayerMovementController
             // through other non-PK players, which is retail's default for
             // ACE's character creation defaults too).
             moverFlags: AcDream.Core.Physics.ObjectInfoState.IsPlayer
-                      | AcDream.Core.Physics.ObjectInfoState.EdgeSlide);
+                      | AcDream.Core.Physics.ObjectInfoState.EdgeSlide,
+            // Fix #42: skip self in FindObjCollisions. Wired by GameWindow
+            // when the local player entity spawns (or stays 0 in tests, in
+            // which case there's no registered ShadowEntry to collide with
+            // anyway).
+            movingEntityId: LocalEntityId);
 
         // L.4-diag (2026-04-30): trace position transitions so we can see
         // whether the body is actually moving frame-to-frame on the steep