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)

include/rendering/character_renderer.hpp

@@ -178,6 +178,9 @@ private:
         bool hasOverrideModelMatrix = false;
         glm::mat4 overrideModelMatrix{1.0f};
 
+        // Bone update throttling (skip frames for distant characters)
+        uint32_t boneUpdateCounter = 0;
+
         // Per-instance bone SSBO (double-buffered per frame)
         VkBuffer boneBuffer[2] = {};
         VmaAllocation boneAlloc[2] = {};