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/core/application.cpp

@@ -731,9 +731,15 @@ void Application::setupUICallbacks() {
 
     // Taxi flight start callback - upload all precached tiles to GPU before flight begins
     gameHandler->setTaxiFlightStartCallback([this]() {
-        if (renderer && renderer->getTerrainManager()) {
-            LOG_INFO("Uploading all precached tiles to GPU before taxi flight...");
+        if (renderer && renderer->getTerrainManager() && renderer->getM2Renderer()) {
+            LOG_INFO("Uploading all precached tiles (terrain + M2 models) to GPU before taxi flight...");
+            auto start = std::chrono::steady_clock::now();
             renderer->getTerrainManager()->processAllReadyTiles();
+            auto end = std::chrono::steady_clock::now();
+            auto durationMs = std::chrono::duration_cast<std::chrono::milliseconds>(end - start).count();
+            uint32_t m2Count = renderer->getM2Renderer()->getModelCount();
+            uint32_t instCount = renderer->getM2Renderer()->getInstanceCount();
+            LOG_INFO("GPU upload completed in ", durationMs, "ms - ", m2Count, " M2 models in VRAM (", instCount, " instances)");
         }
     });