auto-detect MIDI device, drop ALSA sequencer dependency, MPK Mini simulator

midi-bridge.py:
  - Auto-detect rawmidi device (gadget f_midi > env override > fallback)
  - Reconnect on ENODEV (device hotplug)
  - No longer needs MIDI_BRIDGE_DEV env var in common cases

start-synth.sh:
  - Remove all -alsamidi/-midiadddev flags (kernel lacks CONFIG_SND_SEQUENCER)
  - MIDI handled entirely by midi-bridge.service → UDP → netreceive
  - Host mode: same approach, no ALSA sequencer needed

mpk-mini-sim.pd:
  - New MPK Mini Mk II simulator (25 keys, 8 pads, 8 CC knobs)
  - Proper Pd #X obj format for all objects (bng, hsl, t, del, netsend, loadbang)
  - UDP output to board:8081, matching midi-bridge format

system: enable usb-gadget-audio.service at boot, load dwc3 module at boot

system/midi-bridge.service

@@ -1,10 +1,11 @@
 [Unit]
-Description=MIDI rawmidi to Pd UDP bridge (MPK Mini → Pd)
+Description=MIDI rawmidi → Pd UDP bridge (auto-detects gadget/host)
 After=multi-user.target
 
 [Service]
 Type=simple
 ExecStart=/usr/local/bin/midi-bridge.py
+User=root
 Restart=on-failure
 RestartSec=2
 StandardOutput=journal