Keep M2 models permanently in VRAM to eliminate loading hitches

Modern GPUs have 8-16GB VRAM - leverage this to cache all M2 models permanently.

Changes:
- Disabled cleanupUnusedModels() call when tiles unload
- Models now stay in VRAM after initial load, even when tiles unload
- Increased taxi mounting delay from 3s to 5s for more precache time
- Added logging: M2 model count, instance count, and GPU upload duration
- Added debug logging when M2 models are uploaded per tile

This fixes the "building pops up then pause" issue - models were being:
1. Loaded when tile loads
2. Unloaded when tile unloads (behind taxi)
3. Re-loaded when flying through again (causing hitch)

Now models persist in VRAM permanently (few hundred MB for typical session).
First pass loads to VRAM, subsequent passes are instant.

src/game/game_handler.cpp

@@ -262,11 +262,11 @@ void GameHandler::update(float deltaTime) {
             }
         }
 
-        // Mounting delay for taxi (terrain precache time)
+        // Mounting delay for taxi (terrain + M2 model precache time)
         if (taxiMountingDelay_) {
             taxiMountingTimer_ += deltaTime;
-            // 3 second delay for "mounting" animation and terrain precache
-            if (taxiMountingTimer_ >= 3.0f) {
+            // 5 second delay for terrain and M2 models to load and upload to VRAM
+            if (taxiMountingTimer_ >= 5.0f) {
                 taxiMountingDelay_ = false;
                 taxiMountingTimer_ = 0.0f;
                 // Upload all precached tiles to GPU before flight starts