fix(metronome): pickup-aware accents; remove study count-in (core owns it)

[gionnibgud]

What

Follow-up to #22, from a user report: on a 4/4 piano score that opens with a
pickup measure (two eighth notes), the metronome accents fell on the wrong
beats and the study count-in ran a full bar, so the pickup entered where the
downbeat was expected.

Three changes.

1. The pickup is now in the model. The notation JSON already marks it
("pickup": true on measure 0) and core streams the flag through untouched —
_buildScore just never read it. It now maps onto alphaTab’\s isAnacrusis,
so the bar is sized by its content instead of the declared time signature
(verified present in the pinned alphaTab 1.8.2).

2. The metronome no longer accents the pickup’\s click. A pickup is the
tail of an incomplete bar, so the first accent belongs to the first full
measure’\s downbeat. Everything else about the click track is unchanged.

3. The study-mode preroll count-in is removed — toggle, countdown,
pause→play edge tracking (_svWasAudioPaused / _svPrerollResuming), and
the staffview_preroll key. Core already owns a count-in
(countdownBeforeSong), and with both enabled they stacked at song start:
core counted four, then staffview paused and counted again. One count-in,
owned by core, is the right shape — a plugin-local second one has no way to
stay consistent with it. If someone wants study mode to opt out of the global
count-in, that is a core setting, not a re-added plugin countdown.

Study mode is otherwise untouched: it still gate-pauses and resumes, just
without its own clicks.

Note on the fix this does NOT include

The global count-in is still a hardcoded 4 clicks — it ignores the time
signature (a 3/4 song gets 4) and the pickup. That fix belongs in
feedBack/static/js/count-in.js and affects every instrument, so it is a
separate core PR rather than something to work around here.

The converter that produced the bad timeline in the first place is fixed in
got-feedBack/feedBack-plugin-musicxml-import#10 — existing packs need a
re-import for the accents to be right.

Testing

• npm test — 118/118 pass. Two new cases in tests/metronome.test.js
  (pickup: first accent lands on bar 1, not the pickup; no pickup: first-beat
  accent unchanged); the count-in cases are removed with the feature.
• Verified in the running app on the reported song.

Branch note

Rebuilt on current main rather than continuing feat/metronome, which
branched before #21 merged — continuing it would have reverted the
bass-only-cursor fix.

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📒 Files selected for processing (3)

• CHANGELOG.md
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