e84: asio adapter for handshake timers + wall-clock test

The E84StateMachine timers landed last commit but stayed theoretical —
arming was delivered via abstract callbacks the application had to
glue to a real clock.  This commit ships the canonical glue:

- include/secsgem/gem/e84_asio_timers.hpp: header-only
  E84AsioTimers wraps three asio::steady_timers, wires set_timer_handlers
  on attach(), routes async_wait expiry back into fsm.on_timeout().
  detach() cancels everything cleanly.

- tests/test_e84_asio_timers.cpp: four scenarios exercised through a
  real asio::io_context with wall-clock timers — TA1 expiry,
  signal-driven cancel before TA1 fires, TA3 expiry from the
  Transferring state, and detach() halting further transitions.
  These cover the integration the synthetic unit tests in
  test_e84_timers.cpp can't reach.

- INTEGRATION.md §4.6: the vendor-side recipe — create the port,
  set timeouts, make_shared<E84AsioTimers>(...)::attach(), feed signals
  from your I/O bridge.

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

CMakeLists.txt

@@ -132,6 +132,7 @@ add_executable(secsgem_tests
   tests/test_e84.cpp
   tests/test_e84_ports.cpp
   tests/test_e84_timers.cpp
+  tests/test_e84_asio_timers.cpp
   tests/test_e42_formatted_pp.cpp
   tests/test_gem300_scenario.cpp
   tests/test_wire_ceid_emission.cpp