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>

CMakeLists.txt

@@ -48,6 +48,7 @@ add_library(secsgem
   src/secsi/header.cpp
   src/secsi/block.cpp
   src/secsi/protocol.cpp
+  src/secsi/tcp_transport.cpp
   src/gem/control_state.cpp
   src/gem/communication_state.cpp
   src/gem/process_job_state.cpp
@@ -85,6 +86,7 @@ add_executable(secsgem_tests
   tests/test_hsms.cpp
   tests/test_hsms_connection.cpp
   tests/test_secsi.cpp
+  tests/test_secsi_tcp.cpp
   tests/test_control_state.cpp
   tests/test_communication_state.cpp
   tests/test_data_model.cpp