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/TranslucencyKind.cs

@@ -0,0 +1,75 @@
+using DatReaderWriter.Enums;
+
+namespace AcDream.Core.Meshing;
+
+/// <summary>
+/// Categorizes how a sub-mesh should be composited into the frame. Determined
+/// from the Surface.Type flags on the AC dat surface that owns the sub-mesh.
+/// </summary>
+public enum TranslucencyKind
+{
+    /// <summary>Standard opaque. Depth write + test, no blend.</summary>
+    Opaque = 0,
+
+    /// <summary>
+    /// Alpha-keyed (clip-map). Treated as opaque for sorting; the fragment
+    /// shader discards low-alpha fragments. Matches the current rendering of
+    /// doors, windows, and vegetation.
+    /// </summary>
+    ClipMap = 1,
+
+    /// <summary>
+    /// Standard alpha blend: src*a + dst*(1-a).
+    /// Depth-write off, depth-test on. Used for semi-transparent glass,
+    /// water decals, and flame alpha surfaces.
+    /// </summary>
+    AlphaBlend = 2,
+
+    /// <summary>
+    /// Additive blend: src*a + dst. Depth-write off, depth-test on.
+    /// Used for portal swirls, magical glows, and particle effects.
+    /// </summary>
+    Additive = 3,
+
+    /// <summary>
+    /// Inverted alpha blend: src*(1-a) + dst*a. Rare but present in
+    /// the AC dat files.
+    /// </summary>
+    InvAlpha = 4,
+}
+
+public static class TranslucencyKindExtensions
+{
+    // Priority order (highest wins):
+    //   1. Additive   — SurfaceType.Additive (0x10000)
+    //   2. InvAlpha   — SurfaceType.InvAlpha (0x200)
+    //   3. AlphaBlend — SurfaceType.Alpha (0x100) OR SurfaceType.Translucent (0x10)
+    //   4. ClipMap    — SurfaceType.Base1ClipMap (0x04)
+    //   5. Opaque     — everything else
+    //
+    // Note: ACViewer groups Base1ClipMap with the alpha-draw bucket (AlphaSurfaceTypes),
+    // but acdream keeps its existing alpha-discard approach for clip-map surfaces
+    // (they render opaque with per-fragment discard) and introduces a separate
+    // translucent pass only for the genuinely blended surface types.
+
+    /// <summary>
+    /// Maps a <see cref="SurfaceType"/> flags value to the correct
+    /// <see cref="TranslucencyKind"/> for the two-pass render split.
+    /// </summary>
+    public static TranslucencyKind FromSurfaceType(SurfaceType type)
+    {
+        if ((type & SurfaceType.Additive) != 0)
+            return TranslucencyKind.Additive;
+
+        if ((type & SurfaceType.InvAlpha) != 0)
+            return TranslucencyKind.InvAlpha;
+
+        if ((type & (SurfaceType.Alpha | SurfaceType.Translucent)) != 0)
+            return TranslucencyKind.AlphaBlend;
+
+        if ((type & SurfaceType.Base1ClipMap) != 0)
+            return TranslucencyKind.ClipMap;
+
+        return TranslucencyKind.Opaque;
+    }
+}