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.App/Rendering/Shaders/mesh.frag

@@ -5,6 +5,18 @@ out vec4 fragColor;
 
 uniform sampler2D uDiffuse;
 
+// Phase 9.1: translucency kind — matches TranslucencyKind C# enum.
+//   0 = Opaque    — depth write+test, no blend; shader never discards
+//   1 = ClipMap   — alpha-key discard (doors, windows, vegetation)
+//   2 = AlphaBlend — GL blending handles compositing; do NOT discard
+//   3 = Additive  — GL additive blending; do NOT discard
+//   4 = InvAlpha  — GL inverted-alpha blending; do NOT discard
+//
+// Only ClipMap uses the alpha-discard path. AlphaBlend/Additive/InvAlpha
+// rely entirely on the GL blend stage — discarding low-alpha fragments
+// would make semi-transparent surfaces (portals, glows) fully invisible.
+uniform int uTranslucencyKind;
+
 // Phase 3a: simple directional lighting. A single sun direction + ambient term
 // gives scenery and building faces enough differentiation to read as 3D instead
 // of looking like paper cutouts. Hardcoded for now; a later phase can route
@@ -19,8 +31,12 @@ const float DIFFUSE = 0.75;
 
 void main() {
     vec4 sampled = texture(uDiffuse, vTex);
-    // Alpha cutout for doors, windows, vegetation, and other alpha-keyed textures.
-    if (sampled.a < 0.5) discard;
+
+    // Alpha cutout only for clip-map surfaces (doors, windows, vegetation).
+    // Blended surface types (AlphaBlend, Additive, InvAlpha) must NOT
+    // discard here — that would make every semi-transparent pixel invisible
+    // before the blend stage even runs.
+    if (uTranslucencyKind == 1 && sampled.a < 0.5) discard;
 
     vec3 N = normalize(vWorldNormal);
     float ndotl = max(dot(N, SUN_DIR), 0.0);