Add amy_external_midi_output_hook to catch AMY's MIDI out

[bwhitman]

Fixes #895

Adds amy_config.amy_external_midi_output_hook, mirroring the existing amy_external_midi_input_hook, so hosts can catch everything AMY sends over MIDI out and forward it to transports AMY doesn't drive itself — e.g. BLE MIDI on a custom ESP32-S3 board, as requested in the issue.

void my_midi_out_hook(uint8_t *bytes, uint16_t len) {
    // e.g. forward to BT MIDI out
}

amy_config_t amy_config = amy_default_config();
amy_config.amy_external_midi_output_hook = my_midi_out_hook;
amy_start(amy_config);

How it works

• New midi_out_external_hook() helper in amy_midi.c (platform-neutral section) calls the config hook if set.
• Every in-repo midi_out() implementation calls it first, before — and regardless of — any device interface configured in config.midi: the MCU/device layer (UART / USB gadget), the emscripten/WebMIDI build, and macos_midi.m. So the hook fires even with no serial MIDI out pin configured, per the issue.
• The hook sees all of AMY's MIDI output: note on/off forwarding, realtime clock (0xF8/0xFA/0xFC), and sysex responses (zD dumps, transfer frames).
• Hosts that own the whole MIDI layer (AMY_HOST_MIDI, e.g. the VCV plugin) define their own midi_out and can call the helper themselves; nothing changes for them by default.
• Defaults to NULL in amy_default_config(); documented in docs/api.md. Godot builds are unaffected (amy_midi.c/macos_midi.m are excluded there and nothing in the stubs references the helper).

Verified with a clean macOS desktop build (make), which compiles the neutral amy_midi.c section, macos_midi.m, and the new config field together.

🤖 Generated with Claude Code

[github-actions]

🎛️ AMY HW CI (AMYboard bench)

Flashed this PR's AMY (LoadTestChord: 6-voice Juno patch=1, one held note every 2 s) onto the physical AMYboard and measured the smoothed render load as the chord grows — back-to-back with the same sketch built at the PR's merge base, so Δ is this PR's own cost. Each build is flashed once and captured 3 times (board reset between runs); the numbers are per-run means.

✅ PASS — the bench ran the test to completion.

notes held	main @ 73da71e	this PR	Δ
1	1603	1624	+21
2	1800	1824	+24
3	2482	2507	+25
4	2720	2744	+24
5	3465	3493	+28
6	3673	3705	+32

Full chord settled render μs: 3703 (was 3673, Δ +0.8%) (peak 3711, 117 samples, mean of 3 runs)

_{per-run full-chord μs — this PR: 3704 / 3702 / 3703 · main @ 73da71e: 3674 / 3672 / 3672}

⬇️ Artifacts: serial log · load trace · report

_{Self-hosted bench (amyboardci). FAIL means only that the test could not run — the load values are informational, with no threshold and no audio compare. See tools/arduino_loadsweep/.}