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>

tools/e2e/65-song.mjs

@@ -133,6 +133,19 @@ export async function song(t) {
   );
   const id = created.songs[0]?.id;
   const model = () => json(`__gm.song.model(${JSON.stringify(id)})`);
+  // A new song snaps to 1/16; the checks below go by bars, so the grid is set to a bar here.
+  const newGrid = await json('__gm.song.gridAt()');
+  await click(newGrid.x, newGrid.y);
+  await sleep(150);
+  const barEntry = (await json("__gm.locateAll('Bar')")).find((hit) => hit.y > newGrid.y);
+  if (barEntry) await click(barEntry.x, barEntry.y);
+  await sleep(150);
+  const barGrid = await json('__gm.song.gridAt()');
+  check(
+    'a new song snaps to 1/16, and its Grid menu sets a bar',
+    newGrid.text === '1/16' && barGrid.text === 'Bar',
+    JSON.stringify({ newGrid, barGrid }),
+  );
   /**
    * Adds a track under the last one through its head's menu: there is no other way to. Fit first:
    * the tracks a fit shares the height out to leave the one added last below the edge.