pm_e-2: MIDI out — drive external gear (drum module / e-kit) from the editor

engine.js: add an opt-in per-hit hook in scheduleMeterTick — onMeterHit(sound,
time, lvl) — called only if a page defines it (no-op everywhere else). Lets a
page emit MIDI per scheduled hit, in lockstep with the audio scheduler.

pm_e-2.html: a "🎛 MIDI out" header toggle + output-port picker. When on, each
groove hit is sent as a GM drum Note-On (channel 10; note from SOUND_GM e.g.
kick*->36, snare*->38, hat*->42; velocity by accent/normal/ghost) to the chosen
port via output.send([..], ts) with a timestamp derived from the hits audio time
(performance.now() + (time - audioCtx.currentTime)*1000) for tight sync; a note-off
follows 60ms later. Port list prefers a non-PM output (the external gear) and
refreshes on MIDI connect/disconnect. Independent of the local synth + Device
audio; goes green (blue) when on. Web MIDI (Chrome/Edge/Firefox).

Conformance suite still green (engine.js change is in the scheduler, not the codec).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

pm_e-2.html

@@ -298,6 +298,9 @@
             title="Device link (USB-MIDI). Click to connect — Chrome, Edge or Firefox. Turns green and shows the name while a PM device (PM_K-1 / PM_X-1 / PM_G-1) is plugged in; ◎ means none is detected. This only reports the connection — it does not make sound on its own.">◎ connect device</span>
       <button id="midiBtn" class="devctrl"
             title="Device audio — on/off switch. When ON, the notes a connected device sends over USB-MIDI are played through THIS computer's speakers (the device drives the sound, locked to its clock). You can switch it on before plugging in — it doesn't need a device to toggle; plug one in while it's on and it sounds through the computer.">🎹 Device audio</button>
+      <button id="midiOutBtn" class="devctrl"
+            title="MIDI out — on/off switch. When ON, the groove is sent as MIDI notes to external gear (a drum module / e-kit) on the output port chosen at right: GM drum notes on channel 10, scheduled tightly in sync with playback. Independent of the local synth and of Device audio. Web MIDI · Chrome/Edge/Firefox.">🎛 MIDI out</button>
+      <select id="midiOutSel" class="devctrl" title="MIDI output port to drive (your drum module / e-kit)" hidden></select>
       <button id="helpBtn" title="keyboard shortcuts (?)">?</button>
     </div>
   </div>
@@ -1242,7 +1245,7 @@ async function _ensureMidi() {        // MIDI access WITH SysEx (needed to send/
   _midiAccess.onstatechange = _wireMidi; _wireMidi();
   return true;
 }
-function _wireMidi() { for (const inp of _midiInputs()) inp.onmidimessage = onDeviceMidi; updateMidiBtn(); updateDevBadge(); }
+function _wireMidi() { for (const inp of _midiInputs()) inp.onmidimessage = onDeviceMidi; updateMidiBtn(); updateDevBadge(); populateMidiOutPorts(); }
 function updateDevBadge() {   // header badge: lights green while a PM_K-1 / PM_X-1 is connected over USB-MIDI
   const el = $("devBadge"); if (!el) return;
   const dev = _midiAccess ? [..._midiOutputs(), ..._midiInputs()].filter(_isDevicePort) : [];
@@ -1291,6 +1294,51 @@ async function toggleDeviceAudio() {
     ? "Device audio ON.\nMIDI input(s): " + names.join(", ") + "\nPress play on the device — the button pulses green per note."
     : "Armed, but no MIDI input yet. Plug in the PM_K-1 (CircuitPython firmware) — it connects automatically.");
 }
+/* MIDI OUT: send the groove as GM drum notes to a chosen output port, to drive external gear
+   (a drum module / e-kit). Hooks engine.js's per-hit `onMeterHit`, scheduled in sync with audio. */
+const SOUND_GM = {
+  kick:36, kick808:36, kick909:36, snare:38, snare808:38, snare909:38,
+  clap:39, clap808:39, clap909:39, rim:37,
+  hatClosed:42, hat808:42, hat909:42, hatOpen:46, openHat808:46,
+  ride:51, ride909:51, crash:49, crash909:49,
+  tomLow:41, tom808:45, tomMid:45, tomHigh:48, tambourine:54,
+  cowbell:56, cowbell808:56, woodblock:76, jamblock:76, claves:75, beep:37,
+};
+let _midiOutOn = false;
+function _midiOutTarget() {
+  const sel = $("midiOutSel"); if (!sel) return null;
+  return _midiOutputs().find((o) => o.id === sel.value) || null;
+}
+function populateMidiOutPorts() {
+  const sel = $("midiOutSel"); if (!sel) return;
+  const outs = _midiOutputs(), prev = sel.value;
+  sel.innerHTML = outs.length
+    ? outs.map((o) => `<option value="${o.id}">${(o.name || "output").slice(0, 22)}</option>`).join("")
+    : `<option value="">(no MIDI outputs)</option>`;
+  if (outs.some((o) => o.id === prev)) sel.value = prev;               // keep prior choice
+  else { const ext = outs.find((o) => !_isDevicePort(o)); sel.value = ((ext || outs[0] || {}).id) || ""; }  // prefer external gear
+}
+// engine.js calls this per scheduled hit (sound name, audio-context time, level 1/2/3)
+function onMeterHit(sound, time, lvl) {
+  if (!_midiOutOn) return;
+  const out = _midiOutTarget(); if (!out) return;
+  const note = SOUND_GM[sound]; if (note == null) return;
+  const vel = lvl === 2 ? 112 : lvl === 3 ? 45 : 90;                   // accent / ghost / normal
+  const ts = (audioCtx ? performance.now() + (time - audioCtx.currentTime) * 1000 : performance.now());
+  try { out.send([0x99, note, vel], ts); out.send([0x89, note, 0], ts + 60); } catch (_) {}   // note-on, then off (ch10)
+}
+function updateMidiOutBtn() {
+  const b = $("midiOutBtn"), sel = $("midiOutSel"); if (!b) return;
+  if (_midiOutOn) { b.style.color = "#7ab8ff"; b.style.borderColor = "#7ab8ff"; if (sel) sel.hidden = false; }
+  else { b.style.color = "var(--muted)"; b.style.borderColor = "var(--edge)"; if (sel) sel.hidden = true; }
+}
+async function toggleMidiOut() {
+  if (_midiOutOn) { _midiOutOn = false; updateMidiOutBtn(); return; }
+  if (!(await _ensureMidi())) return alert("Driving external MIDI gear needs the Web MIDI API — use Chrome, Edge, or Firefox.");
+  populateMidiOutPorts();
+  if (!_midiOutputs().length) return alert("No MIDI output ports found. Connect your drum module / e-kit (USB-MIDI) and try again.");
+  _midiOutOn = true; updateMidiOutBtn();
+}
 function _queryDeviceVersion() {   // ask the device its firmware version (SysEx 0x02 -> reply 0x03)
   return new Promise((res) => { _verCb = res; _send([0xF0, 0x7D, 0x02, 0xF7]); setTimeout(() => { if (_verCb) { _verCb = null; res(null); } }, 1500); });
 }
@@ -1719,6 +1767,7 @@ $("saveDeviceBtn").addEventListener("click", () => { $("trayMenu").hidden = true
 $("loadDeviceBtn").addEventListener("click", () => { $("trayMenu").hidden = true; loadFromDevice(); });
 $("updateFwBtn").addEventListener("click", () => { $("trayMenu").hidden = true; updateFirmware(); });
 $("midiBtn").addEventListener("click", toggleDeviceAudio);
+$("midiOutBtn").addEventListener("click", toggleMidiOut);
 $("devBadge").addEventListener("click", () => { _ensureMidi().then(updateDevBadge); });
 $("clearLogBtn").addEventListener("click", () => { $("trayMenu").hidden = true; clearLog(); });
 $("resetAllBtn").addEventListener("click", () => { $("trayMenu").hidden = true; resetAll(); });

src/engine.js

@@ -157,6 +157,9 @@ function scheduleMeterTick(m, time) {
   if (!lvl) return;
   const lin = m.gainDb ? Math.pow(10, m.gainDb / 20) : 1;   // per-lane dB gain → linear, applied at schedule time (no stutter)
   playInstrument(m.sound, time, (lvl === 2 ? 1.0 : lvl === 3 ? 0.25 : 0.6) * lin);
+  // opt-in per-hit hook (a page may define onMeterHit to e.g. emit MIDI out to external gear);
+  // (sound name, audio-context time of the hit, dynamic level 1/2/3). No-op on pages that don't set it.
+  if (typeof onMeterHit === "function") onMeterHit(m.sound, time, lvl);
 }
 
 function refBarDur() { return (meters.length ? meters[0].beatsPerBar : 4) * (60 / state.bpm); }