Control other devices/pedals via MIDI

[sastraxi]

Any hardware control can now send its MIDI to an external device port (Source Audio C4, HX Stomp, etc.) instead of the virtual MIDI Through port. Also introduces a pedalboard-load message sending mechanism, allowing pedalboards to ensure external devices are in known state.

Includes #169

What's new

1. The main feaure is per-control routing. Add midi_port: <MIDI device name> to any footswitch, encoder, or analog/expression control, and its MIDI is sent there instead of to mod-ui, falling back if the device is unavailable.
2. An external MIDI manager class provides port enumeration/auto-detect, eager open at routing time, send-with-fallback wrapper, and a pedalboard-load message burst (send_messages_for_pedalboard) for syncing external gear on bundle change.
3. This branch introduces a controller manager class, shared by mod/modhandler, as well as extracting out Current into its own (shared) module.
4. Finally, external controllers show on the LCD, just like they would if they were assigned to a parameter in MOD. Externally-routed controls get a synthetic parameter to do so and work in all hardware versions.
5. Config files, parser, and hardware have been updated to pass-through configuration

Testing

I've used this for a few months, both via init message and scrolling through presets on my C4 with Tweak1. This version has been simplified, decomposed, and tested more thoroughly than that one.

There are MacOS-only loopback tests that open an actual virtual MIDI port and assert bytes travel the full path for every routable control: footswitch press, tweak-encoder rotation, expression movement, plus the pedalboard-load messages and backoff.

Bugs discovered (mod-host)

Connecting/disconnecting an rtmidi output can trigger a crash in mod-host while it attempts to clean up when changing pedalboards. It's relatively rare and I only managed to trigger it by having a pedalboard with 54 (!) dead MIDI port references in the TTL. This will be worked on separately (see mod-audio/mod-host#96)

When ExternalMidiManager._init_port() opens a direct rtmidi port to the C4, it creates
a new ALSA sequencer client (RtMidiOut Client, client 132). JACK sees this as a new
JackPortIsTerminal|JackPortIsPhysical MIDI port and fires mod-host's async
PortRegistration callback, which enqueues a POSTPONED_JACK_MIDI_CONNECT event.

If the user switches pedalboards before that event is drained, mod-host's main thread is
simultaneously running effects_remove() → lilv_instance_free() → dlclose(). The
async connect event and the teardown race on the ALSA/JACK heap, causing corruption.
Subsequent dlclose() calls then SIGSEGV on the corrupted state.

Sonnet 4.6