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/Physics/EntityCollisionFlags.cs

@@ -0,0 +1,67 @@
+namespace AcDream.Core.Physics;
+
+/// <summary>
+/// Per-entity flags driving the retail-faithful PvP / Player /
+/// Impenetrable exemption logic in <c>FindObjCollisions</c>. Decoded
+/// from <c>PublicWeenieDesc._bitfield</c> at <c>CreateObject</c> time.
+///
+/// <para>
+/// Bit positions (verified against
+/// <c>docs/research/named-retail/acclient_2013_pseudo_c.txt:406898-406918</c>
+/// where <c>ACCWeenieObject::IsPK/IsPKLite/IsImpenetrable</c> read directly
+/// from <c>this-&gt;pwd._bitfield</c>):
+/// </para>
+/// <list type="bullet">
+///   <item><c>BF_PLAYER          = 0x8</c>        (bit 3)  → <see cref="IsPlayer"/></item>
+///   <item><c>BF_PLAYER_KILLER   = 0x20</c>       (bit 5)  → <see cref="IsPK"/></item>
+///   <item><c>BF_FREE_PKSTATUS   = 0x200000</c>   (bit 21) → <see cref="IsImpenetrable"/></item>
+///   <item><c>BF_PKLITE_PKSTATUS = 0x2000000</c>  (bit 25) → <see cref="IsPKLite"/></item>
+/// </list>
+///
+/// <para>
+/// <see cref="IsCreature"/> is NOT a PWD bit — retail derives it from
+/// <c>PublicWeenieDesc._type</c> matching <c>ITEM_TYPE_CREATURE</c>
+/// (acclient.h ITEM_TYPE enum). Set at registration time by callers that
+/// already know the item type.
+/// </para>
+/// </summary>
+[Flags]
+public enum EntityCollisionFlags : byte
+{
+    None           = 0x00,
+    /// <summary>Set when <c>BF_PLAYER (0x8)</c> is set in <c>pwd._bitfield</c>.</summary>
+    IsPlayer       = 0x01,
+    /// <summary>Derived from <c>ItemType.Creature</c> on the spawn payload.</summary>
+    IsCreature     = 0x02,
+    /// <summary>Set when <c>BF_PLAYER_KILLER (0x20)</c> is set.</summary>
+    IsPK           = 0x04,
+    /// <summary>Set when <c>BF_PKLITE_PKSTATUS (0x2000000)</c> is set.</summary>
+    IsPKLite       = 0x08,
+    /// <summary>Set when <c>BF_FREE_PKSTATUS (0x200000)</c> is set (a.k.a. "Free" PK status — cannot be PKed).</summary>
+    IsImpenetrable = 0x10,
+}
+
+/// <summary>Helpers to convert raw retail bitfields into <see cref="EntityCollisionFlags"/>.</summary>
+public static class EntityCollisionFlagsExt
+{
+    /// <summary>
+    /// Decode the player/PK/PKLite/Impenetrable bits from a
+    /// <c>PublicWeenieDesc._bitfield</c> value (the WeenieHeader trailer
+    /// field acdream's parser surfaces as <c>ObjectDescriptionFlags</c>).
+    ///
+    /// <para>Bit positions per
+    /// <c>docs/research/named-retail/acclient.h:6431-6463</c>
+    /// (<c>PublicWeenieDesc::BitfieldIndex</c>) and
+    /// <c>acclient_2013_pseudo_c.txt:441868-441890</c>
+    /// (<c>PublicWeenieDesc::SetPlayerKillerStatus</c>).</para>
+    /// </summary>
+    public static EntityCollisionFlags FromPwdBitfield(uint bitfield)
+    {
+        var flags = EntityCollisionFlags.None;
+        if ((bitfield & 0x8u)        != 0) flags |= EntityCollisionFlags.IsPlayer;
+        if ((bitfield & 0x20u)       != 0) flags |= EntityCollisionFlags.IsPK;
+        if ((bitfield & 0x200000u)   != 0) flags |= EntityCollisionFlags.IsImpenetrable;
+        if ((bitfield & 0x2000000u)  != 0) flags |= EntityCollisionFlags.IsPKLite;
+        return flags;
+    }
+}