Pick a pattern's sign in one row, as a song's (#364)

A pattern had two rows for its meter, Beats per bar and Beat, while a
song has had one row Sign since #363. Both now pick from the same list.

- The list of signs and how a sign becomes a meter moved to time.ts.
  Song and pattern share it.
- A pattern keeps its meter in `sign`. The fields `beats` and `beat`
  are gone, a sign that is not in the list counts as 4/4.
- Create pattern in a song hands the song's sign to the new pattern.
- The four patterns of cbbf160bb7e8 were moved to `sign` by hand
  (three 4/4, one 3/4). The other projects only had 4/4, which stays
  the default.

Tests:
- Unit pattern/embedded: 30 green (1 new, 2 rewritten), the
  counter-test reading `beats` again turns 2 red
- e2e 64 + 65 (melektaus-atx): ALL PASSED, 85 checks
- Gate (melektaus-atx): CHECK EXIT 0, 272 files, 4215 tests

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

tests/core/pattern.test.ts

@@ -16,6 +16,7 @@ import {
   patternSequence,
   patternTempo,
 } from '../../src/core/music/pattern';
+import { SIGN_OPTIONS } from '../../src/core/music/time';
 import { schemaForAsset } from '../../src/core/properties/PropertySchema';
 import { makeProject, roundTrip } from '../helpers';
 
@@ -52,8 +53,7 @@ describe('pattern asset (#333)', () => {
       'instrument',
       'bars',
       'grid',
-      'beats',
-      'beat',
+      'sign',
       'tempo',
       'fx',
       'bus',
@@ -70,11 +70,22 @@ describe('pattern asset (#333)', () => {
   it('measures its length in bars of its meter: 3 bars of 6/8 are 4320 ticks', () => {
     const f = makePattern();
     f.set('bars', 3);
-    f.set('beats', 6);
-    f.set('beat', '8');
+    f.set('sign', '6/8');
     expect(patternLength(f.pattern())).toBe(3 * 6 * 240);
   });
 
+  it('picks its meter from the signs a song has too, 4/4 for any other (#364)', () => {
+    const f = makePattern();
+    f.set('sign', '9/8');
+    expect(patternMeter(f.pattern().props)).toEqual({ beats: 9, unit: 8 });
+    f.set('sign', '12/8');
+    expect(patternMeter(f.pattern().props)).toEqual({ beats: 4, unit: 4 });
+    expect(schemaForAsset(f.project, f.pattern()).find((d) => d.key === 'sign')).toMatchObject({
+      kind: 'enum',
+      options: SIGN_OPTIONS,
+    });
+  });
+
   it('holds notes to whole ticks, a tick at least, the keyboard and 0 to 1, sorted, ids unique', () => {
     const held = heldPatternNotes([
       { id: 'b', tick: 480.4, length: 0.2, note: 64, velocity: 2 },