lexus2k / tinyproto / 23957724679

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Implement SREJ (Selective Reject) S-frame support

SREJ allows a receiver to request retransmission of a single specific
frame, unlike REJ which retransmits all frames from N(R) onward.
This is more efficient when only one frame is lost in a window.

Implementation:
- Added HDLC_S_FRAME_TYPE_SREJ (0x0C) constant
- Added TINY_FD_FRAME_SUBTYPE_SREJ to public enum
- RX handler: on SREJ receipt, confirms frames up to N(R) and
  sets srej_req_mask bit for the requested frame number
- TX path: checks srej_req_mask before normal next_ns dispatch,
  retransmits only the specific frame without advancing next_ns
- Uses uint8_t bitmask (perfect for modulo-8 sequence numbers)
- Mask cleared on connect, disconnect, and RSET
- Updated logger to display SREJ subtype

Tests:
- ABM_SREJRetransmitsSingleFrame: basic SREJ retransmit
- ABM_SREJRetransmitsOnlyRequestedFrame: verifies only one frame sent
- ABM_SREJForSecondFrame: SREJ with N(R)=1 and implicit ack
- ABM_REJRetransmitsAllFromNR: contrast test showing REJ sends all
- All 72 tests pass

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

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