A4: SECS-I transport (block protocol + E4 retry FSM)

Adds a complete IO-free SECS-I implementation:

  include/secsgem/secsi/header.hpp   10-byte block header (R/W/E bits)
  include/secsgem/secsi/block.hpp    length + header + body + checksum
  include/secsgem/secsi/protocol.hpp half-duplex FSM (ENQ/EOT/ACK/NAK)
  src/secsi/*                         implementations
  tests/test_secsi.cpp                header, block, multi-block split,
                                      back-to-back FSM drive, RTY,
                                      contention, T2 timeout

The protocol is event-driven (`Event` → `Action` queue), so wiring it
to an asio serial_port is a thin adapter — that lands in the next
commit so this one stays reviewable.

Key design points:
- Master/slave contention: slave yields on simultaneous ENQ (E4 §7.1.4).
- RTY exhaustion raises ActionRaiseError, clears the send queue, resets
  to Idle (no zombie state).
- Multi-block assembler validates contiguous 1..N numbering and exclusive
  E-bit-on-last invariants — rejects malformed sequences with nullopt.
- Block::checksum is exposed publicly for the receive path's verification.

Tests cover the happy path (back-to-back delivery), error paths
(checksum mismatch, short input, oversize body), retries (NAK chain to
exhaustion), and protocol corner cases (contention, T2 timeout).

secsgem-py implements SECS-I block framing but lacks the explicit RTY
state machine; this commit puts the C++ port ahead on transport
correctness.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

COMPLIANCE.md

@@ -40,6 +40,24 @@ Legend:
 
 ---
 
+## 1a. E4 — SECS-I transport (block protocol)
+
+| Item                                  | Status | Spec ref | Notes |
+|---------------------------------------|--------|----------|-------|
+| 10-byte block header (R/W/E bits, system bytes) | ✅ | E4 §6.2 | `secsi::Header` with bit-precise pack/unpack. |
+| Length-prefixed block + 2-byte checksum | ✅   | E4 §6.1, §6.3 | `secsi::Block::encode/decode`. |
+| Multi-block message split / assemble  | ✅     | E4 §7.2.3 | `split_message` / `assemble_message`; E-bit only on the final block. |
+| ENQ/EOT/ACK/NAK handshake             | ✅     | E4 §7.1  | `secsi::Protocol` half-duplex FSM. |
+| RTY retry counter                     | ✅     | E4 §10.2 | Per-block retry budget, exhaust → ActionRaiseError. |
+| T1 inter-character timer hook         | ✅     | E4 §10.1 | Drained in `RecvBlock`; host wires the actual asio timer. |
+| T2 protocol timer hook                | ✅     | E4 §10.1 | Triggers a retry from any send state. |
+| T3 reply timer                        | ⬜     | E4 §10.1 | Driven by the upper layer (same as HSMS). |
+| T4 inter-block timer                  | ⬜     | E4 §10.1 | Multi-block message-gap; FSM emits hook events. |
+| Master/slave contention resolution    | ✅     | E4 §7.1.4 | Slave yields on simultaneous ENQ; master holds. |
+| Serial port wiring (asio)             | ⬜     | —        | FSM is IO-free; a follow-up commit will land the asio integration. |
+
+---
+
 ## 2. E5 — SECS-II encoding
 
 | Item                                  | Status | Spec ref | Notes |