Add lazy piano transcription and optional leased server worker

frontend/piano-core.js

@@ -0,0 +1,59 @@
+/* Compact note sequences and practice ranges, shared by browser and server. */
+(function (root) {
+  'use strict';
+  const MAX_NOTES = 30000;
+  function normalizeNotes(input) {
+    if (!Array.isArray(input) || input.length > MAX_NOTES)
+      throw new Error('Invalid or oversized note sequence');
+    return input
+      .map((note) => {
+        const pitch = Number(note.pitch),
+          start = Number(note.start),
+          end = Number(note.end),
+          velocity = Number(note.velocity);
+        if (
+          !Number.isInteger(pitch) ||
+          pitch < 21 ||
+          pitch > 108 ||
+          !Number.isFinite(start) ||
+          start < 0 ||
+          !Number.isFinite(end) ||
+          end <= start ||
+          end > 3600 ||
+          !Number.isFinite(velocity)
+        )
+          throw new Error('Invalid piano note');
+        return {
+          pitch,
+          start: Math.round(start * 1000) / 1000,
+          end: Math.max(Math.round(end * 1000) / 1000, Math.round(start * 1000) / 1000 + 0.001),
+          velocity: Math.max(1, Math.min(127, Math.round(velocity))),
+          hand: ['left', 'right'].includes(note.hand) ? note.hand : pitch < 60 ? 'left' : 'right',
+        };
+      })
+      .sort((a, b) => a.start - b.start || a.pitch - b.pitch);
+  }
+  function fromBasicPitch(notes) {
+    return normalizeNotes(
+      notes
+        .filter((n) => n.pitchMidi >= 21 && n.pitchMidi <= 108)
+        .map((n) => ({
+          pitch: n.pitchMidi,
+          start: n.startTimeSeconds,
+          end: n.startTimeSeconds + n.durationSeconds,
+          velocity: n.amplitude * 127,
+        })),
+    );
+  }
+  const activeNotes = (notes, time) => notes.filter((n) => n.start <= time && n.end > time);
+  function practiceRange(time, duration) {
+    const start = Math.max(0, Number(time) || 0),
+      end = Math.min(start + 8, Number(duration) || start + 8);
+    return end > start
+      ? { start: Math.round(start * 1000) / 1000, end: Math.round(end * 1000) / 1000 }
+      : null;
+  }
+  const api = { normalizeNotes, fromBasicPitch, activeNotes, practiceRange, MAX_NOTES };
+  if (typeof module !== 'undefined') module.exports = api;
+  else root.PianoCore = api;
+})(typeof window !== 'undefined' ? window : globalThis);