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

include/rendering/camera_controller.hpp

@@ -89,6 +89,7 @@ public:
     void setMountHeightOffset(float offset) { mountHeightOffset_ = offset; }
     void setExternalFollow(bool enabled) { externalFollow_ = enabled; }
     void setExternalMoving(bool moving) { externalMoving_ = moving; }
+    void setFacingYaw(float yaw) { facingYaw = yaw; }  // For taxi/scripted movement
     void clearMovementInputs();
 
     // For first-person player hiding