MidiStream fans one aseqdump subprocess out to N web subscribers (SSE), reusing
midimon.parse_line + enrich (one parsing source of truth); reader runs only
while watched (lazy start / stop on last unsubscribe, so no MIDI port held idle).
parse_ports is pure + tested. NOT yet wired into server/UI — see resume task.
Parked mid-feature: server SSE endpoint + dashboard panel still to do.
