I2: Router::dispatch_with_s9 helper + end-to-end S9F3/F5 tests

The S9F3/F5 fallback was previously inlined in apps/secs_server.cpp;
this commit lifts it onto Router as a template helper and adds two
focused tests asserting the wire behaviour against a real back-to-
back HSMS Connection pair.

  template <typename EmitFn, typename HeaderProvider>
  std::optional<Message> dispatch_with_s9(emit, header, msg);

The helper does the has_handler / has_handler_for_stream check and
calls the supplied emit function with S9F3 (unknown stream) or S9F5
(unknown function in known stream).  The header_provider returns the
optional MHEAD bytes — keeping the helper free of any direct
Connection coupling.

Tests:
  - SUT registered only for S1F1; peer sends S1F5 -> SUT replies
    S9F5 to the peer.
  - SUT registered only for S1F1; peer sends S7F19 -> SUT replies
    S9F3 to the peer.

Closes Tranche I — SML parser and the auto-S9F* fallback closeout
both verified end-to-end.

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

CMakeLists.txt

@@ -112,6 +112,7 @@ add_executable(secsgem_tests
   tests/test_cem_objects.cpp
   tests/test_modules.cpp
   tests/test_sml.cpp
+  tests/test_s9_fallback.cpp
 )
 target_link_libraries(secsgem_tests PRIVATE secsgem doctest::doctest)
 target_compile_definitions(secsgem_tests PRIVATE