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/core/song-markers.test.ts

@@ -0,0 +1,103 @@
+import { describe, expect, it } from 'vitest';
+import { CreateAssetCommand } from '../../src/core/commands/assetCommands';
+import { SetSongMarkersCommand } from '../../src/core/commands/songCommands';
+import type { SongAsset } from '../../src/core/model/Asset';
+import {
+  changedMarker,
+  heldSongMarkers,
+  newMarker,
+  nextMarkerName,
+  withoutMarker,
+} from '../../src/core/music/markers';
+import { makeProject, roundTrip } from '../helpers';
+
+function makeSong() {
+  const f = makeProject();
+  const create = new CreateAssetCommand('song', 'Tune');
+  f.stack.execute(create);
+  const song = () => f.project.assets.find((a) => a.id === create.asset.id) as SongAsset;
+  return { ...f, song };
+}
+
+describe('song markers (#338)', () => {
+  it('holds markers to whole ticks from 0, a name and ids of their own, in the order of their ticks', () => {
+    const held = heldSongMarkers([
+      { id: 'b', name: ' Boss ', tick: 3840.4 },
+      { id: 'a', name: 'Intro', tick: 0 },
+      { id: 'a', name: '', tick: 1920 },
+      { id: 'c', name: 'Before', tick: -1 },
+      { id: 'd', name: 'Nowhere' },
+      'no marker',
+      null,
+    ]);
+    expect(held.map((m) => [m.name, m.tick])).toEqual([
+      ['Intro', 0],
+      ['Marker 3', 1920],
+      ['Boss', 3840],
+    ]);
+    expect(new Set(held.map((m) => m.id)).size).toBe(3);
+    expect(held.find((m) => m.tick === 1920)!.id).not.toBe('a');
+    expect(heldSongMarkers(undefined)).toEqual([]);
+  });
+
+  it('keeps markers on one tick in the order they came', () => {
+    const held = heldSongMarkers([
+      { id: 'x', name: 'Second', tick: 960 },
+      { id: 'y', name: 'First', tick: 0 },
+      { id: 'z', name: 'Third', tick: 960 },
+    ]);
+    expect(held.map((m) => m.name)).toEqual(['First', 'Second', 'Third']);
+  });
+
+  it('names a new marker by the first number not taken', () => {
+    expect(nextMarkerName([])).toBe('Marker 1');
+    const markers = heldSongMarkers([
+      { name: 'Marker 1', tick: 0 },
+      { name: 'Marker 3', tick: 10 },
+    ]);
+    expect(nextMarkerName(markers)).toBe('Marker 2');
+    const made = newMarker(markers, 479.6);
+    expect(made).toMatchObject({ name: 'Marker 2', tick: 480 });
+    expect(newMarker(markers, -5, '  Drop ')).toMatchObject({ name: 'Drop', tick: 0 });
+  });
+
+  it('changes or takes away one marker and leaves the others', () => {
+    const markers = heldSongMarkers([
+      { id: 'a', name: 'A', tick: 0 },
+      { id: 'b', name: 'B', tick: 960 },
+    ]);
+    expect(changedMarker(markers, 'b', { tick: 1920 })).toEqual([markers[0], { ...markers[1], tick: 1920 }]);
+    expect(changedMarker(markers, 'a', { name: 'Start' })[0]).toEqual({ id: 'a', name: 'Start', tick: 0 });
+    expect(withoutMarker(markers, 'a')).toEqual([markers[1]]);
+  });
+
+  it('writes the markers as one step, leaves the key out without one and takes it back on undo', () => {
+    const f = makeSong();
+    expect(f.song().markers).toBeUndefined();
+    const one = [{ id: 'm', name: 'Drop', tick: 1920.2 }];
+    expect(SetSongMarkersCommand.changes(f.song(), one)).toBe(true);
+    f.stack.execute(new SetSongMarkersCommand(f.song().id, one, 'Add marker'));
+    expect(f.song().markers).toEqual([{ id: 'm', name: 'Drop', tick: 1920 }]);
+    expect(SetSongMarkersCommand.changes(f.song(), one)).toBe(false);
+    f.stack.execute(new SetSongMarkersCommand(f.song().id, [], 'Delete marker'));
+    expect('markers' in f.song()).toBe(false);
+    f.stack.undo();
+    expect(f.song().markers).toEqual([{ id: 'm', name: 'Drop', tick: 1920 }]);
+    f.stack.undo();
+    expect('markers' in f.song()).toBe(false);
+  });
+
+  it('keeps the markers through saving and loading', () => {
+    const f = makeSong();
+    const markers = [
+      { id: 'a', name: 'Intro', tick: 0 },
+      { id: 'b', name: 'Boss', tick: 7680 },
+    ];
+    f.stack.execute(new SetSongMarkersCommand(f.song().id, markers));
+    const back = roundTrip(f.project).assets.find((a) => a.id === f.song().id) as SongAsset;
+    expect(back.markers).toEqual(markers);
+    const none = makeSong();
+    const plain = roundTrip(none.project).assets.find((a) => a.id === none.song().id) as SongAsset;
+    expect('markers' in plain).toBe(false);
+  });
+});