Fix taxi mount orientation and eliminate tile loading hitches

Fixes two critical taxi flight issues:

1. Mount orientation now correctly faces flight direction:
   - Prevent camera controller from updating facingYaw during taxi (externalFollow_ check)
   - Taxi orientation callback system updates mount rotation from spline tangent
   - Initial orientation set when flight starts
   - Smooth Catmull-Rom spline interpolation for natural curved paths

2. Eliminate frame hitches from tile loading during flight:
   - New taxiFlightStartCallback uploads ALL precached tiles to GPU before flight begins
   - Previously tiles loaded async during 3s mount delay but uploaded 1/frame during flight
   - Now processAllReadyTiles() blocks briefly after mount delay to batch upload everything
   - Combined with 2.0s terrain update interval and aggressive culling for smooth flight

Additional optimizations:
   - Aggressive taxi culling: skip models <15 units, all foliage/trees, underwater objects
   - Max render distance reduced to 150 units during taxi
   - Movement heartbeat packets disabled during taxi (server controls position)
   - Reduced taxi speed from 32 to 18 units/sec to prevent streaming overload

src/rendering/m2_renderer.cpp

@@ -1644,7 +1644,8 @@ void M2Renderer::render(const Camera& camera, const glm::mat4& view, const glm::
     lastDrawCallCount = 0;
 
     // Adaptive render distance: keep longer tree/foliage visibility to reduce pop-in.
-    const float maxRenderDistance = (instances.size() > 600) ? 320.0f : 2800.0f;
+    // During taxi, use very short render distance to prevent loading hitches
+    const float maxRenderDistance = onTaxi_ ? 150.0f : (instances.size() > 600) ? 320.0f : 2800.0f;
     const float maxRenderDistanceSq = maxRenderDistance * maxRenderDistance;
     const float fadeStartFraction = 0.75f;
     const glm::vec3 camPos = camera.getPosition();
@@ -1713,10 +1714,20 @@ void M2Renderer::render(const Camera& camera, const glm::mat4& view, const glm::
 
         const M2ModelGPU& model = *currentModel;
 
-        // Skip small models when on taxi (performance optimization)
-        // Small props/foliage aren't visible from flight altitude anyway
-        if (onTaxi_ && model.boundRadius < 3.0f) {
-            continue;
+        // Aggressive culling during taxi for smooth flight
+        if (onTaxi_) {
+            // Skip all small/medium models (props, foliage, decorations)
+            if (model.boundRadius < 15.0f) {
+                continue;
+            }
+            // Skip all foliage and trees (even large ones cause hitching during load)
+            if (model.collisionNoBlock || model.collisionTreeTrunk) {
+                continue;
+            }
+            // Skip underwater objects (water is opaque from altitude)
+            if (instance.position.z < -5.0f) {
+                continue;
+            }
         }
 
         // Distance-based fade alpha for smooth pop-in (squared-distance, no sqrt)