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/Meshing/GfxObjSubMesh.cs

@@ -9,4 +9,13 @@ namespace AcDream.Core.Meshing;
 public sealed record GfxObjSubMesh(
     uint SurfaceId,
     Vertex[] Vertices,
-    uint[] Indices);
+    uint[] Indices)
+{
+    /// <summary>
+    /// How this sub-mesh should be composited into the frame.
+    /// Populated from Surface.Type flags at upload time (requires a DatCollection).
+    /// Defaults to <see cref="TranslucencyKind.Opaque"/> so offline fixtures
+    /// that don't supply dat access compile and pass unchanged.
+    /// </summary>
+    public TranslucencyKind Translucency { get; init; } = TranslucencyKind.Opaque;
+}