Enhance VRInputHandler with stale catch-up ramp for smoother pose recovery

[Zachary359]

Summary

• Add per-hand stale-input tracking for VR pose targets.
  • Detect six or more consecutive identical XR input frames.
  • On recovery, distribute the gap between the latest XR input and previous output across subsequent frames, avoiding an abrupt target jump.
  • Keep normal pose updates unmodified when no stale-input recovery is active.
• Publish every valid XR pose callback instead of suppressing unchanged poses through the pose-change threshold.
• Reset stale-input ramp state with existing VR mode, input, and target-stream reset paths, preventing residual state from crossing control transitions.
• Add startup and debug logging for stale-input catch-up behavior.
• Document continuous target publishing and stale-input catch-up ramp recovery in VR_INPUT_HANDLER_README.md.