feat(editor): clarify tempo map guidance

screen.js

@@ -10375,12 +10375,19 @@ function _tempoMapDraw(w, h) {
     ctx.font = '11px sans-serif';
     ctx.textAlign = 'left';
     ctx.textBaseline = 'middle';
-    ctx.fillText(
-        `Tempo Map — ${measures.length} measures · drag a pole to retime · `
-        + `right-click: insert/delete · [ ]: time signature`,
-        LABEL_W + 6, hudY);
+    ctx.fillText(_tempoMapHudTextPure(measures.length, w), LABEL_W + 6, hudY);
 }
 
+/* @pure:tempo-map-guidance:start */
+function _tempoMapHudTextPure(measureCount, width) {
+    const n = Number.isFinite(Number(measureCount)) ? Number(measureCount) : 0;
+    if (Number(width) < 760) {
+        return `Tempo Map - ${n} measures - right-click sync point: BPM / signature`;
+    }
+    return `Tempo Map - ${n} measures - drag poles to retime - right-click sync point: BPM / signature/delete - right-click grid: insert`;
+}
+/* @pure:tempo-map-guidance:end */
+
 function _ensureTempoSignatureControl() {
     let wrap = document.getElementById('editor-tempo-sig-wrap');
     if (wrap) return wrap;
@@ -10414,7 +10421,7 @@ function _ensureTempoMapButton() {
         btn.type = 'button';
         btn.textContent = '🎵 Tempo Map';
         btn.className = 'px-3 py-1 bg-dark-600 hover:bg-dark-500 rounded text-xs font-medium hidden';
-        btn.title = 'Open the EOF-style tempo-map editor';
+        btn.title = 'Open Tempo Map to edit sync points, BPM, and time signatures';
         btn.onclick = () => {
             // Finalize any in-progress canvas drag before switching
             // modes — commit a moved sync-point / drum drag (don't
@@ -10609,8 +10616,8 @@ function _tempoMapOnMouseDown(e, x, y) {
     draw();
 }
 
-// Right-click in tempo-map mode: insert a sync point on open grid, or
-// delete / change the time signature of the sync point under the cursor.
+// Right-click in tempo-map mode: insert on open grid, or edit/delete
+// the sync point under the cursor.
 function _tempoMapOnContextMenu(e) {
     const { x, y } = getMousePos(e);
     const menu = document.getElementById('editor-context-menu');

tests/tempo_map_guidance.test.js

@@ -0,0 +1,50 @@
+'use strict';
+/*
+ * Tempo-map guidance helper tests for screen.js.
+ *
+ * Run: node tests/tempo_map_guidance.test.js
+ */
+const fs = require('fs');
+const path = require('path');
+const assert = require('assert');
+
+const src = fs.readFileSync(path.join(__dirname, '..', 'screen.js'), 'utf8');
+const m = src.match(/\/\* @pure:tempo-map-guidance:start \*\/[\s\S]*?\/\* @pure:tempo-map-guidance:end \*\//);
+if (!m) {
+    console.error('FAIL: @pure:tempo-map-guidance block not found in screen.js');
+    process.exit(1);
+}
+
+const api = new Function(
+    '"use strict";' + m[0] + '\nreturn { _tempoMapHudTextPure };'
+)();
+
+let pass = 0;
+let fail = 0;
+function t(name, fn) {
+    try { fn(); pass++; console.log('  ok   ' + name); }
+    catch (e) { fail++; console.error('  FAIL ' + name + ': ' + e.message); }
+}
+
+t('uses compact guidance on narrow canvases', () => {
+    const text = api._tempoMapHudTextPure(12, 640);
+    assert.ok(text.includes('12 measures'));
+    assert.ok(text.includes('right-click sync point'));
+    assert.ok(text.includes('BPM / signature'));
+    assert.ok(!text.includes('right-click grid'));
+});
+
+t('uses full guidance when there is room', () => {
+    const text = api._tempoMapHudTextPure(24, 960);
+    assert.ok(text.includes('24 measures'));
+    assert.ok(text.includes('drag poles to retime'));
+    assert.ok(text.includes('BPM / signature/delete'));
+    assert.ok(text.includes('right-click grid: insert'));
+});
+
+t('normalizes invalid measure counts to zero', () => {
+    assert.ok(api._tempoMapHudTextPure('bad', 960).includes('0 measures'));
+});
+
+console.log(`\n${pass} passed, ${fail} failed`);
+process.exit(fail ? 1 : 0);