Fix glow sprite flashing, move fadeAlpha to push constants, throttle character bones

- Glow sprites now use dedicated vertex buffer (glowVB_) separate from
  M2 particle buffer to prevent data race when renderM2Particles()
  overwrites glow data mid-flight
- Move fadeAlpha from shared material UBO to per-draw push constants,
  eliminating cross-instance alpha race on non-double-buffered UBOs
- Smooth adaptive render distance transitions to prevent pop-in/out
  at instance count thresholds (1000/2000)
- Distance-tiered character bone throttling: near (<30u) every frame,
  mid (30-60u) every 3rd, far (60-120u) every 6th frame
- Skip weapon instance animation updates (transforms set by parent bones)

assets/shaders/m2.frag.glsl

@@ -34,6 +34,7 @@ layout(location = 1) in vec3 Normal;
 layout(location = 2) in vec2 TexCoord;
 layout(location = 3) flat in vec3 InstanceOrigin;
 layout(location = 4) in float ModelHeight;
+layout(location = 5) in float vFadeAlpha;
 
 layout(location = 0) out vec4 outColor;
 
@@ -175,16 +176,16 @@ void main() {
     float fogFactor = clamp((fogParams.y - dist) / (fogParams.y - fogParams.x), 0.0, 1.0);
     result = mix(fogColor.rgb, result, fogFactor);
 
-    float outAlpha = texColor.a * fadeAlpha;
+    float outAlpha = texColor.a * vFadeAlpha;
     // Cutout materials should not remain partially transparent after discard,
     // otherwise foliage cards look view-dependent.
     if (alphaTest != 0 || colorKeyBlack != 0) {
-        outAlpha = fadeAlpha;
+        outAlpha = vFadeAlpha;
     }
     // Foliage cutout should stay opaque after alpha discard to avoid
     // view-angle translucency artifacts.
     if (alphaTest == 2 || alphaTest == 3) {
-        outAlpha = 1.0 * fadeAlpha;
+        outAlpha = 1.0 * vFadeAlpha;
     }
     outColor = vec4(result, outAlpha);
 }

assets/shaders/m2.vert.glsl

@@ -19,6 +19,7 @@ layout(push_constant) uniform Push {
     int texCoordSet;
     int useBones;
     int isFoliage;
+    float fadeAlpha;
 } push;
 
 layout(set = 2, binding = 0) readonly buffer BoneSSBO {
@@ -37,6 +38,7 @@ layout(location = 1) out vec3 Normal;
 layout(location = 2) out vec2 TexCoord;
 layout(location = 3) flat out vec3 InstanceOrigin;
 layout(location = 4) out float ModelHeight;
+layout(location = 5) out float vFadeAlpha;
 
 void main() {
     vec4 pos = vec4(aPos, 1.0);
@@ -86,6 +88,7 @@ void main() {
 
     InstanceOrigin = push.model[3].xyz;
     ModelHeight = pos.z;
+    vFadeAlpha = push.fadeAlpha;
 
     gl_Position = projection * view * worldPos;
 }