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>

src/AcDream.Core.Net/WorldSession.cs

@@ -56,7 +56,14 @@ public sealed class WorldSession : IDisposable
         string? Name,
         uint? ItemType,
         CreateObject.ServerMotionState? MotionState,
-        uint? MotionTableId);
+        uint? MotionTableId,
+        // Commit A 2026-04-29 — live-entity collision plumbing.
+        // PhysicsState: retail acclient.h:2815 (ETHEREAL_PS=0x4,
+        // IGNORE_COLLISIONS_PS=0x10, HAS_PHYSICS_BSP_PS=0x10000, ...).
+        // ObjectDescriptionFlags: retail PWD._bitfield (acclient.h:6431-6463)
+        // — drives IsPlayer/IsPK/IsPKLite/IsImpenetrable for PvP gating.
+        uint? PhysicsState = null,
+        uint? ObjectDescriptionFlags = null);
 
     /// <summary>Fires when the session finishes parsing a CreateObject.</summary>
     public event Action<EntitySpawn>? EntitySpawned;
@@ -648,7 +655,9 @@ public sealed class WorldSession : IDisposable
                         parsed.Value.Name,
                         parsed.Value.ItemType,
                         parsed.Value.MotionState,
-                        parsed.Value.MotionTableId));
+                        parsed.Value.MotionTableId,
+                        parsed.Value.PhysicsState,
+                        parsed.Value.ObjectDescriptionFlags));
                 }
             }
             else if (op == DeleteObject.Opcode)