Size notes by either end and pan before bar one in music tabs

A round of small fixes to the Pattern and Song tabs: what the ruler
and a pan do in both, and a few things the tabs carried that were in
the way.

- A middle-drag can take bar 1 up to half the grid or lanes in. The
  time before it is drawn dark on the ruler, the grid and the lanes.
  Nothing lands there: a click draws no note, a drop shows no frame.
- A press or drag on the ruler puts the start on the nearest grid
  line, no longer on the one before the pointer.
- A new song has one track instead of four. The "+ Track" row under
  the heads is gone, a head's menu adds tracks above or below. A song
  still keeps at least one, so that menu is always there.
- A note can be sized by its left end too: the start goes to the
  nearest grid line and the end stays put, never before tick 0.
- An end of a note under the pointer lights up and shows a resize
  cursor, as on an item of the Song tab.

Tests:
- Unit: piano-roll, song-edit, song, song-play 52 tests. Two new
  cases for the left end and the pan, the song fixtures add their
  tracks themselves now.
- e2e 64+65 on melektaus-atx: ALL PASSED - 50 checks (+2).
  e2e 65 adds its tracks through the head's menu and clicks the
  ruler three quarters into a bar, which only the nearest line
  passes.
- Gate on melektaus-atx: 251 files, 3909 tests, CHECK EXIT 0.
- A probe shot of the piano roll panned in: the time before bar 1
  dark, the left end of a note lit.

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

src/editor/debug/songHooks.ts

@@ -27,7 +27,6 @@ export function songHooks({ state, editor }: HookContext) {
       laneAt: (tick: number, track: number) => timeline.lanePoint(tick, track),
       itemAt: (itemId: string, part?: 'start' | 'body' | 'end') => timeline.itemPoint(itemId, part),
       headAt: (track: number, part: HeadPart, share?: number) => timeline.headPoint(track, part, share),
-      addTrackAt: () => timeline.addTrackPoint(),
       rulerAt: (tick: number) => timeline.rulerPoint(tick),
       loopAt: (tick: number) => timeline.loopPoint(tick),
       view: () => timeline.viewInfo(),