feat(physics): live-entity collision plumbing (Commit A)

Plumbing-only foundation for the upcoming live-entity (NPC / monster
/ player) collision port. No behavior change — the new fields default
to zero/None so the 5 existing static-entity Register call sites in
GameWindow.cs are untouched.

Wire layer:
- CreateObject parser now surfaces PhysicsState (acclient.h:2815 —
  ETHEREAL_PS=0x4, IGNORE_COLLISIONS_PS=0x10, HAS_PHYSICS_BSP_PS=0x10000,
  ...) which the parser previously dropped at line ~337 with a bare
  `pos += 4`.
- CreateObject parser now surfaces ObjectDescriptionFlags (the retail
  PWD._bitfield trailer per acclient.h:6431-6463), where
  acclient_2013_pseudo_c.txt:406898-406918 ACCWeenieObject::IsPK /
  IsPKLite / IsImpenetrable read bits 5 / 25 / 21 directly. Previously
  read-and-discarded.
- WorldSession.EntitySpawn carries both new fields through to subscribers.

Physics layer:
- New `EntityCollisionFlags` enum (IsPlayer / IsCreature / IsPK /
  IsPKLite / IsImpenetrable) + `FromPwdBitfield` helper. Bit
  positions verified against retail's SetPlayerKillerStatus (
  acclient_2013_pseudo_c.txt:441868-441890) which maps
  PKStatusEnum→bitfield exactly: PK=0x4→bit5, PKLite=0x40→bit25,
  Free=0x20→bit21.
- `ShadowEntry` extended with `State` (raw PhysicsState bits) +
  `Flags` (decoded EntityCollisionFlags). Backward-compatible — all
  five existing landblock-entity Register call sites omit them.
- `ShadowObjectRegistry.UpdatePosition(entityId, pos, rot, ...)` —
  fast-path for the 5–10 Hz UpdatePosition (0xF748) stream the server
  emits per visible entity. Reuses the entry's existing shape +
  state + flags. Mirrors retail's CPhysicsObj::SetPosition
  (acclient_2013_pseudo_c.txt:284276) which keeps the same shape and
  re-registers cell membership.
- `ObjectInfoState` adds `IsPK = 0x800` and `IsPKLite = 0x1000`
  matching retail's OBJECTINFO::state bits (acclient.h:6190-6194).
  Used by Commit C's PvP exemption gate.

Tests:
- `EntityCollisionFlagsTests` — 7 tests covering empty / each bit
  alone / PK+player combo / unrelated-bit ignore.
- `ShadowObjectRegistryTests` — 5 new tests: UpdatePosition moves
  entry to new cell, preserves State/Flags, unregistered no-op,
  Register stores State/Flags, defaults are zero/None.
- `CreateObjectTests` — 3 new tests verifying PhysicsState + PWD
  bitfield (with PK / PKLite bit cases) parse and surface.

1454 → 1454 + 15 = covered by suite. dotnet build + dotnet test
green.

Foundation for Commit B (live-entity registration) and Commit C
(PvP exemption block in FindObjCollisions).

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

tests/AcDream.Core.Tests/Physics/ShadowObjectRegistryTests.cs

@@ -177,4 +177,79 @@ public class ShadowObjectRegistryTests
         var objs = reg.GetObjectsInCell(cellId);
         Assert.Contains(objs, e => e.EntityId == 99u);
     }
+
+    // -----------------------------------------------------------------------
+    // UpdatePosition — Commit A of 2026-04-29 live-entity collision port
+    // -----------------------------------------------------------------------
+
+    [Fact]
+    public void UpdatePosition_MovedEntity_NewCellOccupied()
+    {
+        // Entity starts at local (12, 12) — cell (0,0).
+        var reg = new ShadowObjectRegistry();
+        reg.Register(42u, 0x01000010u, new Vector3(12f, 12f, 50f), Quaternion.Identity, 0.5f, OffX, OffY, LbId);
+
+        // Move to local (60, 12) — cell (2, 0). Same landblock.
+        reg.UpdatePosition(42u, new Vector3(60f, 12f, 50f), Quaternion.Identity, OffX, OffY, LbId);
+
+        // Old cell empty, new cell holds the entity.
+        Assert.Empty(reg.GetObjectsInCell(LbId | 1u));   // cell (0,0)
+        var newCell = reg.GetObjectsInCell(LbId | 17u);  // cell (2,0) → 2*8+0+1
+        Assert.Single(newCell);
+        Assert.Equal(42u, newCell[0].EntityId);
+        Assert.Equal(1, reg.TotalRegistered);            // not duplicated
+    }
+
+    [Fact]
+    public void UpdatePosition_PreservesFlags()
+    {
+        // Register with PK flags + PhysicsState; UpdatePosition must keep them.
+        var reg = new ShadowObjectRegistry();
+        reg.Register(50u, 0x01000011u, new Vector3(12f, 12f, 50f), Quaternion.Identity,
+                     0.5f, OffX, OffY, LbId,
+                     state: 0x4u, // ETHEREAL_PS
+                     flags: EntityCollisionFlags.IsPlayer | EntityCollisionFlags.IsPK);
+
+        reg.UpdatePosition(50u, new Vector3(60f, 12f, 50f), Quaternion.Identity, OffX, OffY, LbId);
+
+        var newCell = reg.GetObjectsInCell(LbId | 17u);
+        Assert.Single(newCell);
+        Assert.Equal(0x4u, newCell[0].State);
+        Assert.Equal(EntityCollisionFlags.IsPlayer | EntityCollisionFlags.IsPK, newCell[0].Flags);
+    }
+
+    [Fact]
+    public void UpdatePosition_UnregisteredEntity_NoOp()
+    {
+        var reg = new ShadowObjectRegistry();
+        // Should not throw, should not register a new entity.
+        reg.UpdatePosition(99u, new Vector3(12f, 12f, 50f), Quaternion.Identity, OffX, OffY, LbId);
+        Assert.Equal(0, reg.TotalRegistered);
+    }
+
+    [Fact]
+    public void Register_WithStateAndFlags_StoredOnEntry()
+    {
+        var reg = new ShadowObjectRegistry();
+        reg.Register(60u, 0x01000012u, new Vector3(12f, 12f, 50f), Quaternion.Identity,
+                     0.5f, OffX, OffY, LbId,
+                     state: 0x10u, // IGNORE_COLLISIONS_PS
+                     flags: EntityCollisionFlags.IsImpenetrable);
+
+        var entry = reg.GetObjectsInCell(LbId | 1u)[0];
+        Assert.Equal(0x10u, entry.State);
+        Assert.Equal(EntityCollisionFlags.IsImpenetrable, entry.Flags);
+    }
+
+    [Fact]
+    public void Register_DefaultStateAndFlags_AreZeroAndNone()
+    {
+        var reg = new ShadowObjectRegistry();
+        reg.Register(70u, 0x01000013u, new Vector3(12f, 12f, 50f), Quaternion.Identity,
+                     0.5f, OffX, OffY, LbId);
+
+        var entry = reg.GetObjectsInCell(LbId | 1u)[0];
+        Assert.Equal(0u, entry.State);
+        Assert.Equal(EntityCollisionFlags.None, entry.Flags);
+    }
 }