hsms-gs: worked integration example + INTEGRATION.md §7

The codebase has supported HSMS-GS since the original landing
(test_hsms_gs.cpp covers the wire-level Select.req-per-session
walk-list, the per-session Reject(EntityNotSelected) behaviour,
and session-routed data dispatch).  But the documentation said
exactly one line about it ("Connection::add_session(device_id)
registers extra sessions on one TCP socket") and there was no
end-to-end test using the Server/Client API customers actually
build against.

INTEGRATION.md §7 is a new section showing the realistic pattern:

- Server-side: register the primary session via Server::Config,
  then `add_session` for the second MES in the on_connection
  callback.  Per-session message handler + selected handler so
  each MES gets its own router (or its own per-session data view
  over a shared EquipmentDataModel).
- Active-mode: same `add_session` on the host-side Connection
  for multi-tool fleet controllers.
- Equipment-initiated push: pick the session_id when sending
  unsolicited primaries (S5F1, S6F11, S10F1).
- Pointer to the wire tests + the new integration test for
  customers who want to see the failure modes.

tests/test_hsms_gs_integration.cpp drives two MES sessions
(device_id 1 + 2) through the Server/Client API end to end:
- Both sessions complete Select.req independently
- S1F1 sent on each session returns a distinct MDLN
  ("EQUIP-SESS-1" vs "EQUIP-SESS-2"), proving per-session
  dispatch routes correctly
- Per-session router fires exactly once per session, no
  cross-talk

Pre-existing §§8-10 in INTEGRATION.md got bumped to §§9-11 to
make room.

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

CMakeLists.txt

@@ -148,6 +148,7 @@ add_executable(secsgem_tests
   tests/test_persistence_upgrade.cpp
   tests/test_config_validate.cpp
   tests/test_metrics_prometheus.cpp
+  tests/test_hsms_gs_integration.cpp
 )
 target_link_libraries(secsgem_tests PRIVATE secsgem doctest::doctest)
 target_compile_definitions(secsgem_tests PRIVATE