A5: SECS-I-over-TCP convenience layer

Wires the SECS-I Protocol FSM behind an asio TCP socket so the block
protocol can run over loopback without serial hardware.  Mirrors
secsgem-py's `secsitcp/` adapter — useful for back-to-back simulators
and CI without a serial device.

Adds:
  include/secsgem/secsi/tcp_transport.hpp
  src/secsi/tcp_transport.cpp
  tests/test_secsi_tcp.cpp

The transport:
- Splits outgoing SECS-II messages into blocks (transparent multi-block).
- Accumulates incoming blocks until end_block=true, then assembles and
  delivers as a single SECS-II message — same surface as the HSMS
  Connection's MessageHandler.
- Drives T1 / T2 timers from asio steady_timer; T3/T4 stay upper-layer
  per the FSM contract.
- Auto-allocates monotonic system bytes per send.

Tests cover single-block delivery, multi-block reassembly (700-byte
ASCII body spanning multiple SECS-I blocks), and bidirectional exchange.

This closes Tranche A (catch-up to secsgem-py wire/transport surface).

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

COMPLIANCE.md

@@ -54,7 +54,8 @@ Legend:
 | 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. |
+| Serial port wiring (asio)             | ⬜     | —        | FSM is IO-free; serial integration is a wiring follow-up. |
+| TCP tunnel for testing                | ✅     | —        | `secsi::TcpTransport` wraps the FSM behind an asio TCP socket; mirrors secsgem-py's `secsitcp/`. |
 
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