Optimize M2 update loop: skip static doodads, incremental spatial index

- Split M2 instances into fast-path index lists (animated, particle-only,
  particle-all, smoke) to avoid iterating all 46K instances per frame
- Cache model flags (hasAnimation, disableAnimation, isSmoke, etc.) on
  M2Instance struct to eliminate per-frame hash lookups
- Replace full rebuildSpatialIndex on position/transform updates with
  incremental grid cell remove+add, preventing 8.5ms/frame rebuild cost
- Advance animTime for all instances (texture UV animation) but only
  compute bones and particles for the ~3K that need it

M2_UPDATE: 10.7ms → 2.0ms, FPS: 35 → 55-59

src/core/application.cpp

@@ -2,6 +2,7 @@
 #include "core/coordinates.hpp"
 #include <unordered_set>
 #include <cmath>
+#include <chrono>
 #include "core/spawn_presets.hpp"
 #include "core/logger.hpp"
 #include "core/memory_monitor.hpp"
@@ -47,7 +48,6 @@
 
 #include <SDL2/SDL.h>
 // GL/glew.h removed — Vulkan migration Phase 1
-#include <chrono>
 #include <cstdlib>
 #include <algorithm>
 #include <cctype>