Add mount pitch and roll for realistic taxi flight animation

Flying mounts now tilt and bank realistically during taxi flights:
- Pitch (up/down): calculated from spline tangent's z-component (altitude change)
- Roll (banking): proportional to turn rate, clamped to ~40 degrees
- Yaw: existing horizontal orientation from spline direction

Implementation:
- Added mountPitch_ and mountRoll_ to Renderer (radians)
- Updated TaxiOrientationCallback to pass yaw, pitch, roll
- Calculate pitch using asin(tangent.z) for altitude tilt
- Calculate roll from yaw change rate: -orientDiff * 2.5, clamped to ±0.7 rad
- Applied to mount rotation: glm::vec3(pitch, roll, yaw)

This fixes the "weirdness" where mounts flew sideways or without natural banking.

include/rendering/renderer.hpp

@@ -129,6 +129,7 @@ public:
     // Mount rendering
     void setMounted(uint32_t mountInstId, float heightOffset);
     void setTaxiFlight(bool onTaxi) { taxiFlight_ = onTaxi; }
+    void setMountPitchRoll(float pitch, float roll) { mountPitch_ = pitch; mountRoll_ = roll; }
     void clearMount();
     bool isMounted() const { return mountInstanceId_ != 0; }
 
@@ -274,6 +275,8 @@ private:
     // Mount state
     uint32_t mountInstanceId_ = 0;
     float mountHeightOffset_ = 0.0f;
+    float mountPitch_ = 0.0f;  // Up/down tilt (radians)
+    float mountRoll_ = 0.0f;   // Left/right banking (radians)
     bool taxiFlight_ = false;
 
     bool terrainEnabled = true;