physics: implement player-controlled flying mount physics

When CAN_FLY + FLYING movement flags are both set (flying mounts, Druid
Flight Form), the CameraController now uses 3D pitch-following movement
instead of ground physics:
- Forward/back follows the camera's 3D look direction (ascend when
  looking up, descend when looking down)
- Space = ascend vertically, X (while mounted) = descend
- No gravity, no grounding, no jump coyote time
- Fall-damage checks suppressed (grounded=true)

Also wire up all remaining server movement state flags to CameraController:
- Feather Fall: cap terminal velocity at -2 m/s
- Water Walk: clamp to water surface, skip swim entry
- Flying: 3D movement with no gravity

All states synced each frame from GameHandler via isPlayerFlying(),
isFeatherFalling(), isWaterWalking(), isGravityDisabled().

include/game/game_handler.hpp

@@ -1166,6 +1166,11 @@ public:
     bool isWaterWalking() const {
         return (movementInfo.flags & static_cast<uint32_t>(MovementFlags::WATER_WALK)) != 0;
     }
+    bool isPlayerFlying() const {
+        const uint32_t flyMask = static_cast<uint32_t>(MovementFlags::CAN_FLY) |
+                                 static_cast<uint32_t>(MovementFlags::FLYING);
+        return (movementInfo.flags & flyMask) == flyMask;
+    }
     void dismount();
 
     // Taxi / Flight Paths