fix(render): shader reserved-word + defensive SkyRenderer dat reads

Two runtime blockers discovered after merging the sky/weather/lighting
branch:

1. GLSL reserved word: mesh.frag + mesh_instanced.frag used \`int active\`
   as a local. On GLSL ES / some drivers \`active\` is a reserved identifier
   and compile fails hard (\"ERROR: 0:38: 'active' : Reserved word\").
   Renamed to \`activeLights\`.

2. SkyRenderer.EnsureMeshUploaded called DatCollection.Get<GfxObj>
   without the _datLock that wraps the streaming pipeline's dat reads.
   DatBinReader has shared buffer state; concurrent reads race and
   throw ArgumentOutOfRangeException from Vec2Duv.Unpack deep in the
   mesh parse. Wrapped both Get<GfxObj> and GfxObjMesh.Build in
   try/catch and cache a null entry on failure so we don't retry every
   frame and crash the render loop. Full fix would plumb _datLock into
   the sky renderer, left as a TODO.

Client now stable end-to-end — in-world, spawn stream flowing,
animation + audio + sky + light UBO all live.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/AcDream.App/Rendering/Shaders/mesh.frag

@@ -52,9 +52,9 @@ layout(std140, binding = 1) uniform SceneLighting {
 // light" look relies on crisp boundaries.
 vec3 accumulateLights(vec3 N, vec3 worldPos) {
     vec3 lit = uCellAmbient.xyz;
-    int active = int(uCellAmbient.w);
+    int activeLights = int(uCellAmbient.w);
     for (int i = 0; i < 8; ++i) {
-        if (i >= active) break;
+        if (i >= activeLights) break;
 
         int kind = int(uLights[i].posAndKind.w);
         vec3 Lcol = uLights[i].colorAndIntensity.xyz * uLights[i].colorAndIntensity.w;

src/AcDream.App/Rendering/Shaders/mesh_instanced.frag

@@ -35,9 +35,9 @@ layout(std140, binding = 1) uniform SceneLighting {
 
 vec3 accumulateLights(vec3 N, vec3 worldPos) {
     vec3 lit = uCellAmbient.xyz;
-    int active = int(uCellAmbient.w);
+    int activeLights = int(uCellAmbient.w);
     for (int i = 0; i < 8; ++i) {
-        if (i >= active) break;
+        if (i >= activeLights) break;
 
         int kind = int(uLights[i].posAndKind.w);
         vec3 Lcol = uLights[i].colorAndIntensity.xyz * uLights[i].colorAndIntensity.w;