feat(net): Phase 6.6 — parse UpdateMotion (0xF74C) into MotionUpdated event

Server sends UpdateMotion whenever an entity's motion state changes:
NPCs starting a walk cycle, creatures switching to a combat stance,
doors opening, a player waving, etc. Phase 6.1-6.4 already handles
rendering different (stance, forward-command) pairs for the INITIAL
CreateObject, but without this message NPCs freeze in whatever pose
they spawned with and never transition to walking/fighting.

Added UpdateMotion.TryParse with the same ServerMotionState the
CreateObject path uses, reached via a slightly different outer
layout (guid + instance seq + header'd MovementData; the MovementData
starts with the 8-byte sequence/autonomous header this time rather
than being preceded by a length field). Only the (stance, forward-
command) pair is extracted — same subset CreateObject grabs.

WorldSession dispatches MotionUpdated(guid, state) when a 0xF74C
body parses successfully. The App-side wiring (guid→entity lookup
and AnimatedEntity cycle swap) is intentionally deferred to a
separate commit because it touches GameWindow which is currently
being edited by the Phase 9.1 translucent-pass work.

89 Core.Net tests (was 83, +6 for UpdateMotion coverage).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/AcDream.Core.Net/WorldSession.cs

@@ -58,6 +58,22 @@ public sealed class WorldSession : IDisposable
     /// <summary>Fires when the session finishes parsing a CreateObject.</summary>
     public event Action<EntitySpawn>? EntitySpawned;
 
+    /// <summary>
+    /// Payload for <see cref="MotionUpdated"/>: the server guid of the entity
+    /// whose motion changed and its new server-side stance + forward command.
+    /// The renderer uses these to drive per-entity cycle switching.
+    /// </summary>
+    public readonly record struct EntityMotionUpdate(
+        uint Guid,
+        CreateObject.ServerMotionState MotionState);
+
+    /// <summary>
+    /// Fires when the session parses a 0xF74C UpdateMotion game message.
+    /// Subscribers can look up the entity by guid and transition its
+    /// animation cycle to the new (stance, forward-command) pair.
+    /// </summary>
+    public event Action<EntityMotionUpdate>? MotionUpdated;
+
     /// <summary>Raised every time the state machine transitions.</summary>
     public event Action<State>? StateChanged;
 
@@ -243,6 +259,22 @@ public sealed class WorldSession : IDisposable
                         parsed.Value.MotionTableId));
                 }
             }
+            else if (op == UpdateMotion.Opcode)
+            {
+                // Phase 6.6: the server sends UpdateMotion (0xF74C) whenever an
+                // already-spawned entity changes its motion state — NPCs
+                // starting a walk cycle, creatures entering combat, doors
+                // opening, etc. We dispatch a lightweight event with the
+                // new (stance, forward-command) pair so the animation
+                // system can swap the entity's cycle.
+                var motion = UpdateMotion.TryParse(body);
+                if (motion is not null)
+                {
+                    MotionUpdated?.Invoke(new EntityMotionUpdate(
+                        motion.Value.Guid,
+                        motion.Value.MotionState));
+                }
+            }
         }
     }