fix(count-in): follow the song's meter and its pickup measure (#1029)

The count-in always clicked exactly four beats, so a 3/4 song was counted
in 4/4, and a song opening with a pickup (anacrusis) had the pickup enter
where the downbeat belonged — putting the player a beat ahead all song.

Bar length now comes from the song_timeline beats already on the highway
(measure >= 0 marks downbeats), so no new plumbing: the time_signatures
map is streamed to plugins rather than stored in the frontend. A first bar
shorter than that meter shortens the count by its length — a 1-beat pickup
in 4/4 counts "1 2 3" and the music enters on 4.

Bar length is the mode of the downbeat gaps, not the first gap, so a
pickup's own short gap can't be read as the meter; the beats trailing the
last downbeat count as a candidate too, or a song of pickup + one bar
offers only the pickup's gap. Pickup shortening is scoped to the song's
first bar — a short bar elsewhere is a meter change, and is counted by its
own length instead. Songs without beats (pre-chart, minigames, synthetic
highways) still get four.

Applies to both count-in paths: loop wrap / section practice, and the
start-of-song 'Countdown before song' setting.

Signed-off-by: gionnibgud <gionnibgud@gmail.com>
Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

tests/js/loop_restart.test.js

@@ -88,6 +88,10 @@ function buildSandbox() {
         playClick: () => {},
         showCountOverlay: () => {},
         hideCountOverlay: () => {},
+        // beginCount sizes the count to the bar at loop A; the wrap-path
+        // assertions below don't depend on how many clicks it decides on.
+        // Covered directly in count_in_beats.test.js.
+        countInBeats: () => 4,
 
         // Stubbed DOM access. Anything querying for a button just gets a
         // permissive object that ignores writes.