fix: add 60-second grace period to M2 model cleanup

Models that lose all instances are no longer immediately evicted from
GPU memory. Instead they get a 60-second grace period, preventing the
thrash cycle where GO models (barrels, chests, herbs) were evicted
every 5 seconds and re-loaded when the same object type respawned.

include/rendering/m2_renderer.hpp

@@ -13,6 +13,7 @@
 #include <string>
 #include <optional>
 #include <random>
+#include <chrono>
 #include <future>
 
 namespace wowee {
@@ -434,6 +435,9 @@ private:
     void* glowVBMapped_ = nullptr;
 
     std::unordered_map<uint32_t, M2ModelGPU> models;
+    // Grace period for model cleanup: track when a model first became instanceless.
+    // Models are only evicted after 60 seconds with no instances.
+    std::unordered_map<uint32_t, std::chrono::steady_clock::time_point> modelUnusedSince_;
     std::vector<M2Instance> instances;
 
     // O(1) dedup: key = (modelId, quantized x, quantized y, quantized z) → instanceId