Let a song carry named markers on a strip in the song tab (#338)

Stage 1 of #338. Game sync needs named places in a song: an event when
the playhead passes one, and a switch that jumps to one. This stage
adds the markers themselves. Games do nothing with them yet.

- A song has markers, each with id, name and tick. A song without
  markers stores nothing for them. Two markers may share tick and
  name, since nodes will name them by id.
- Markers change neither the sound nor the bake fingerprint.
- The song tab draws a strip under the ruler. A marker has a green
  flag with its name and a faint line through the tracks. Tracks and
  lanes sit 18 px lower. The ruler of the pattern tab is unchanged.
- Double click on an empty place adds a marker on the grid, with Alt
  or Shift on the exact tick. Dragging moves it in one undo step.
- A click on a marker sets the start of playback there. A double click
  renames it in the name dialog of the hierarchy groups.
- The context menu offers Play from here, Rename and Delete marker,
  on an empty place Add marker. A tooltip shows the full name. The
  wheel over the strip zooms time like the ruler.
- New agent commands add_song_marker, change_song_marker and
  remove_song_marker. get_asset lists them under markers.

Tests:
- song-markers in core 6 and editor 4, 1 agent test, with the touched
  song, agent and MCP tests 175/175
- 9 counter-probes, all compile, all red
- e2e 65 on atx: ALL PASSED, 70 checks, 3 new in 65g3. Counter-probe
  where a dragged marker is not taken over: 65g3 red
- gate on atx: CHECK EXIT 0, 332 files, 5093 tests

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/editor/agent-music.test.ts

@@ -224,6 +224,45 @@ describe('songs', () => {
     expect(noFx.text).toContain('is no Sound FX asset');
   });
 
+  it('adds, renames, moves and removes markers of a song (#338), and get_asset lists them', async () => {
+    const fixture = makeSpriteScene();
+    const { Theme } = await made(fixture, { Theme: 'song' });
+    const song = () => byId<SongAsset>(fixture, Theme);
+    const done = await run(fixture, [
+      { command: 'add_song_marker', args: { asset: Theme, tick: 7680, name: 'Boss' }, as: 'boss' },
+      { command: 'add_song_marker', args: { asset: Theme, tick: 0 } },
+      { command: 'change_song_marker', args: { asset: Theme, marker: '$boss', tick: 3840 } },
+    ]);
+    expect(done.result.isError).toBeUndefined();
+    expect(song().markers!.map(({ name, tick }) => [name, tick])).toEqual([
+      ['Marker 1', 0],
+      ['Boss', 3840],
+    ]);
+    const listed = (await read(fixture, Theme)).markers as { id: string; name: string; tick: number }[];
+    expect(listed.map((m) => m.name)).toEqual(['Marker 1', 'Boss']);
+
+    const boss = song().markers![1]!.id;
+    const blank = await run(fixture, [
+      { command: 'change_song_marker', args: { asset: Theme, marker: boss, name: '  ' } },
+    ]);
+    expect(blank.text).toContain('A marker needs a name');
+    const renamed = await run(fixture, [
+      { command: 'change_song_marker', args: { asset: Theme, marker: boss, name: 'Fight' } },
+    ]);
+    expect(renamed.result.isError).toBeUndefined();
+    expect(song().markers![1]).toEqual({ id: boss, name: 'Fight', tick: 3840 });
+
+    const gone = await run(fixture, [
+      { command: 'remove_song_marker', args: { asset: Theme, marker: boss } },
+    ]);
+    expect(gone.result.isError).toBeUndefined();
+    expect(song().markers!.map((m) => m.name)).toEqual(['Marker 1']);
+    const missing = await run(fixture, [
+      { command: 'remove_song_marker', args: { asset: Theme, marker: boss } },
+    ]);
+    expect(missing.text).toContain(`Theme has no marker ${boss}`);
+  });
+
   it('sets the volume lane of a track in tick order with its easings, refuses what is out of range, and clears it with []', async () => {
     const fixture = makeSpriteScene();
     const { Theme, Bend } = await made(fixture, { Theme: 'song', Bend: 'curve' });

tests/editor/song-markers.test.ts

@@ -0,0 +1,59 @@
+import { describe, expect, it } from 'vitest';
+import { heldSongMarkers } from '../../src/core/music/markers';
+import {
+  MARKER_FLAG,
+  MARKER_REACH,
+  MARKER_STRIP,
+  markerAt,
+  markerNameX,
+  markerSpans,
+  SONG_TOP,
+} from '../../src/editor/song/markerView';
+import { RULER_HEIGHT } from '../../src/editor/music/timeView';
+
+/** A tick a pixel, from x 100; every name 6 pixels a letter. */
+const VIEW = { panX: 100, pxPerTick: 1 };
+const measure = (name: string) => name.length * 6;
+
+describe('the marker strip of the song tab (#338)', () => {
+  it('lies between the ruler and the tracks', () => {
+    expect(SONG_TOP).toBe(RULER_HEIGHT + MARKER_STRIP);
+  });
+
+  it('draws a marker at its tick, as wide as its flag and name, but not into the next one', () => {
+    const markers = heldSongMarkers([
+      { id: 'a', name: 'Intro', tick: 0 },
+      { id: 'b', name: 'A long name', tick: 40 },
+      { id: 'c', name: '', tick: 300 },
+    ]);
+    const spans = markerSpans(VIEW, markers, measure);
+    expect(spans.map((s) => s.x)).toEqual([100, 140, 400]);
+    // Room to the next one is 40 pixels, less 2: the name of "Intro" (30) and the flag fit just so.
+    expect(spans[0]!.w).toBe(38);
+    expect(spans[1]!.w).toBe(MARKER_FLAG + 3 + measure('A long name'));
+    expect(spans[2]!.w).toBe(MARKER_FLAG + 3 + measure('Marker 3'));
+    expect(markerNameX(spans[0]!)).toBe(100 + MARKER_FLAG + 3);
+  });
+
+  it('keeps the flag of a marker with another one close behind it', () => {
+    const markers = heldSongMarkers([
+      { id: 'a', name: 'Close', tick: 0 },
+      { id: 'b', name: 'By', tick: 2 },
+    ]);
+    expect(markerSpans(VIEW, markers, measure)[0]!.w).toBe(MARKER_FLAG);
+  });
+
+  it('takes the marker near its line or on its flag and name, the later one where they overlap', () => {
+    const markers = heldSongMarkers([
+      { id: 'a', name: 'Intro', tick: 0 },
+      { id: 'b', name: 'B', tick: 6 },
+    ]);
+    const spans = markerSpans(VIEW, markers, measure);
+    expect(markerAt(spans, 100 - MARKER_REACH)).toBe('a');
+    expect(markerAt(spans, 100 - MARKER_REACH - 1)).toBeNull();
+    expect(markerAt(spans, 103)).toBe('b');
+    expect(markerAt(spans, 106 + MARKER_FLAG + 3 + 6)).toBe('b');
+    expect(markerAt(spans, 106 + MARKER_FLAG + 3 + 7)).toBeNull();
+    expect(markerAt([], 100)).toBeNull();
+  });
+});