The pen plane from design/pen-tablet-input.md, protocol side only (the P1 uinput
tablet injector will consume it): 0xCC kind 0x05 carries batches of state-full
PenSamples (pressure, polar tilt + azimuth, barrel roll, hover distance, eraser
tool, barrel buttons); PenTracker diffs samples into injector transitions so a
lost datagram self-heals; HOST_CAP_PEN (0x10) gates sending and stays
unadvertised until a real backend exists. C ABI: PunktfunkPenSample +
punktfunk_connection_send_pen, documented in docs/embedding-the-c-abi.md §8.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>