lexus2k / tinyproto / 23957027949

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Implement P/F bit enforcement in ABM mode and UI frame support

Phase 2 HDLC ABM compliance improvements:

P/F Bit Enforcement:
- ABM mode: P bit only set on U-frames and keep-alive RR polls
- NRM mode: unchanged (P always set for marker passing)
- P-to-F propagation: RR response mirrors P bit from command
- Regular I-frames and RR acks no longer carry P bit in ABM

UI (Unnumbered Information) Frames:
- Added HDLC_U_FRAME_TYPE_UI constant (0x00)
- Public API: tiny_fd_send_ui_packet_to(), tiny_fd_send_ui_packet()
- RX handler: calls on_read_ui_cb callback when UI frame received
- UI frames work in any connection state (connectionless)
- Uses s_queue (service queue) for TX

Tests:
- Updated 6 existing tests for P/F bit expectation changes
- Added 7 new tests: P/F enforcement (4), UI send/receive (3)
- All 68 tests pass

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

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