Snap a new song to 1/16, and route new assets as the others (#365)

A new song snapped to whole bars, too coarse to lay out a beat. And a
new or duplicated sound, instrument, pattern or song went to the
master alone, so every one had to be linked into the buses by hand
again.

- A song without a grid of its own snaps to 1/16. The songs in the
  user's project have Bar written in and keep it.
- A new or duplicated sound, instrument, pattern or song gets the links
  of the one whose targets most others of its type share, at their
  volume. On a tie the latest of them decides, so a single song decides
  for the next one.
- Where nothing of its type is linked yet, it goes as most of all that
  plays, and where nothing is, to the master. A new bus still goes to
  the master alone.
- Assets made at once are matched against what stood before, not
  against each other. A project from before the routing still opens
  with everything on the master.
- The song tab has a gridAt hook. e2e 65 checks the new grid and sets
  Bar, which its checks go by.

Tests:

- Unit master-routing (1 new) and song (1 rewritten): 26 green. Counter
  checks (rule off, grid back to Bar) red.
- e2e 65 (melektaus-atx): ALL PASSED, 58 checks (1 new).
- Gate (melektaus-atx): CHECK EXIT 0, 273 files, 4226 tests.

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

src/editor/debug/songHooks.ts

@@ -33,6 +33,7 @@ export function songHooks({ state, editor }: HookContext) {
       items: () => timeline.itemsDrawn(),
       playAt: () => ({ ...centreOf(songEditor.playButton), text: songEditor.playButton.text }),
       fitAt: () => centreOf(songEditor.fitButton),
+      gridAt: () => ({ ...centreOf(songEditor.gridButton), text: songEditor.gridButton.text }),
       playing: () => songEditor.isPlaying,
       /** Record (#342): its button, whether a take is being made, and the input's level. */
       recordAt: () => ({ ...centreOf(songEditor.recordButton), text: songEditor.recordButton.text }),