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] = {};

include/rendering/m2_renderer.hpp

@@ -371,6 +371,11 @@ private:
     ::VkBuffer m2ParticleVB_ = VK_NULL_HANDLE;
     VmaAllocation m2ParticleVBAlloc_ = VK_NULL_HANDLE;
     void* m2ParticleVBMapped_ = nullptr;
+    // Dedicated glow sprite vertex buffer (separate from particle VB to avoid data race)
+    static constexpr size_t MAX_GLOW_SPRITES = 2000;
+    ::VkBuffer glowVB_ = VK_NULL_HANDLE;
+    VmaAllocation glowVBAlloc_ = VK_NULL_HANDLE;
+    void* glowVBMapped_ = nullptr;
 
     std::unordered_map<uint32_t, M2ModelGPU> models;
     std::vector<M2Instance> instances;
@@ -477,6 +482,7 @@ private:
     // Cached camera state from update() for frustum-culling bones
     glm::vec3 cachedCamPos_ = glm::vec3(0.0f);
     float cachedMaxRenderDistSq_ = 0.0f;
+    float smoothedRenderDist_ = 1000.0f;  // Smoothed render distance to prevent flickering
 
     // Thread count for parallel bone animation
     uint32_t numAnimThreads_ = 1;