Seed the e2e fixture where the API actually looks

[thcp]

The bug

tests/e2e/seed.py wrote both peaks.json and beats.json into the job root.
The pipeline writes them under stems/, and that is the only place the
endpoints look — so both 404'd for the whole life of the browser suite:

Artifact	Fixture wrote	API reads	Consequence
beats.json	<job>/beats.json	<job>/stems/beats.json	Studio reported "No beat grid for this track"; every click-track control stayed disabled
peaks.json	<job>/peaks.json	<job>/stems/peaks.json	Studio fell back to decoding every stem for its waveforms

So the click track, the count-in and the grid editor could not be covered in a
browser at all — while seed.py, at a glance, looked like it covered them.
And the precomputed-peaks path that every real track takes was never exercised;
the fixture silently tested the full-decode fallback instead.

The grid now also carries the shape beatgrid.py emits rather than an invented
one, bars included — without bar marks the accent mode degrades to "Auto
(none found)" and the detected-meter path never runs.

What it unlocks

tests/e2e/click-track.spec.mjs, covering what was previously untestable:

• the beat grid reaching the studio at all (the test that would have caught this)
• the click toggling on and off
• count-in persisting across a reload, from the store rather than the DOM
• the rate control reporting the tempo it is actually clicking (/2 → 60 BPM,
  x2 → 240 BPM)
• the accent choice, including "accenting 4/4 from the detected downbeat",
  which only works because bars is now present
• the grid editor opening and closing from the button, a second click, and Done

A note on the helper

waitForClickTrack is separate from openStudio deliberately. openStudio
returns once the transport reports a duration, but the metronome is built later
— after the audio engine is up and the grid has been fetched. Acting before
then hits a null metronome, where the rate and accent controls silently
no-op: the click looks present and does nothing. That is what made the rate
test fail on the first run, and it seemed worth naming rather than papering
over with a sleep.

Verification

25/25 e2e pass (19 existing + 6 new). Confirmed by request log that
stems/peaks.json and beats now return 200 where they previously 404'd, and
that the "no peaks.json; using full-decode engine" warning is gone.