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.Net.Tests/Messages/CreateObjectTests.cs

@@ -23,10 +23,63 @@ public sealed class CreateObjectTests
         Assert.Equal((uint)ItemType.Creature, parsed.Value.ItemType);
     }
 
+    // -----------------------------------------------------------------------
+    // Commit A of 2026-04-29 live-entity collision port:
+    // PhysicsState (post-flags u32) + ObjectDescriptionFlags (PWD bitfield)
+    // must be surfaced for downstream collision registration.
+    // -----------------------------------------------------------------------
+
+    [Fact]
+    public void TryParse_PhysicsState_Parsed()
+    {
+        // ETHEREAL_PS = 0x4 + IGNORE_COLLISIONS_PS = 0x10 → 0x14
+        byte[] body = BuildMinimalCreateObjectWithWeenieHeader(
+            guid: 0x50000003u, name: "GhostNpc",
+            itemType: (uint)ItemType.Creature,
+            physicsState: 0x14u);
+
+        var parsed = CreateObject.TryParse(body);
+
+        Assert.NotNull(parsed);
+        Assert.Equal(0x14u, parsed!.Value.PhysicsState);
+    }
+
+    [Fact]
+    public void TryParse_ObjectDescriptionFlags_PlayerKillerBitsSurface()
+    {
+        // BF_PLAYER (0x8) | BF_PLAYER_KILLER (0x20) → a PK player.
+        byte[] body = BuildMinimalCreateObjectWithWeenieHeader(
+            guid: 0x50000004u, name: "+PkPlayer",
+            itemType: (uint)ItemType.Creature,
+            objectDescriptionFlags: 0x8u | 0x20u);
+
+        var parsed = CreateObject.TryParse(body);
+
+        Assert.NotNull(parsed);
+        Assert.Equal(0x28u, parsed!.Value.ObjectDescriptionFlags);
+    }
+
+    [Fact]
+    public void TryParse_ObjectDescriptionFlags_PkLiteBit()
+    {
+        // BF_PLAYER (0x8) | BF_PKLITE_PKSTATUS (0x2000000) → a PK-lite player.
+        byte[] body = BuildMinimalCreateObjectWithWeenieHeader(
+            guid: 0x50000005u, name: "+PklPlayer",
+            itemType: (uint)ItemType.Creature,
+            objectDescriptionFlags: 0x8u | 0x2000000u);
+
+        var parsed = CreateObject.TryParse(body);
+
+        Assert.NotNull(parsed);
+        Assert.Equal(0x2000008u, parsed!.Value.ObjectDescriptionFlags);
+    }
+
     private static byte[] BuildMinimalCreateObjectWithWeenieHeader(
         uint guid,
         string name,
-        uint itemType)
+        uint itemType,
+        uint physicsState = 0,
+        uint objectDescriptionFlags = 0)
     {
         var bytes = new List<byte>();
         WriteU32(bytes, CreateObject.Opcode);
@@ -38,9 +91,9 @@ public sealed class CreateObjectTests
         bytes.Add(0);
         bytes.Add(0);
 
-        // PhysicsData: no flags, empty physics state, then 9 sequence stamps.
-        WriteU32(bytes, 0);
+        // PhysicsData: physics flags = 0, then PhysicsState u32, then 9 seq stamps.
         WriteU32(bytes, 0);
+        WriteU32(bytes, physicsState);
         for (int i = 0; i < 9; i++)
             WriteU16(bytes, 0);
         Align4(bytes);
@@ -51,7 +104,7 @@ public sealed class CreateObjectTests
         WritePackedDword(bytes, 0x1234); // WeenieClassId
         WritePackedDword(bytes, 0);      // IconId via known-type writer
         WriteU32(bytes, itemType);
-        WriteU32(bytes, 0);              // ObjectDescriptionFlags
+        WriteU32(bytes, objectDescriptionFlags);
         Align4(bytes);
 
         return bytes.ToArray();