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>
