Add amy_config.midi_thru: echo MIDI in to MIDI out (#738)

[bwhitman]

Closes #738.

What

Adds an amy_config_t.midi_thru flag. When set, AMY echoes every incoming MIDI byte straight to MIDI out, in addition to processing it normally. Off by default.

amy_config_t c = amy_default_config();
c.midi_thru = 1;          // enable at startup
amy_start(c);
// ...or toggle at runtime:
amy_global.config.midi_thru = 1;

How

Forwarding is done byte-by-byte at the top of convert_midi_bytes_to_messages() — the single function that all MIDI inputs (UART via esp_poll_midi/on_pico_uart_rx, USB via check_tusb_midi) funnel through.

Forwarding inside the parse loop (rather than blasting the whole input buffer) keeps USB correct: USB MIDI always delivers fixed 3-byte groups, padded with junk for <3-byte messages. The existing if(usb) i = len+1 early-exits stop the loop before those padding bytes, so only the real MIDI bytes of a USB message are passed through. UART input has no padding and forwards in full.

Notes

• No new symbols added to amy_midi.c (only the body of an existing function changed), so no Godot platform-stub updates are needed.
• On Tulip this is exposed to Python as tulip.amy_midi_thru(True/False) (companion tulipcc PR).
• Web targets are out of scope for now; the field is platform-agnostic and stays off unless explicitly enabled.

Test

• make test — all 94 tests pass (midi_thru is off by default; no behavior change to existing paths).
• Verified amyboard (ESP32-S3) firmware compiles with the change.

Docs updated: docs/api.md (config table) and docs/midi.md (new "MIDI Thru" section).