[LeadcodeDev]

Closes #349.

The diagnosis in the issue is wrong, the symptom is real

The issue reads the flat bars as the synthesised score (#331) being incompatible
with the audio-reactive components, since that is the combination it was found on.
It rules out two causes and lands on "check source_fingerprint and
decode_audio_file against the synth's own WAV format".

None of that is it. Two measurements:

	result
full render of the issue's scenario (synthesised score)	bars move — 2011 to 6655 lit pixels across four instants
still of the same scene with an explicit WAV src	flat, identical at every instant

The score was never the variable. analyze_scenario_audio runs from the two
encoders and the studio, and from nowhere else — still and sheet went
straight to painting, so every audio-reactive component in an exported frame sat
at its min.

The fix

still and sheet shared a three-step preamble with the encoders, and it had
drifted: load the custom fonts, prefetch the icons, and now analyse the audio.
preload_scenario_assets is that preamble in one place, so the next thing a frame
needs before it can be painted has one place to be added rather than three.

On the issue's own repro

120 BPM, a kick on every beat, audio_spectrum with 48 bars:

	lit pixels	topmost
before, every instant	1472	508 (the min_height floor)
after, between kicks (t=0.30)	5172	479
after, on a kick (t=0.05)	10788	420

What the still looks like now

A real low-frequency spectrum of the kick, where it used to be a flat dotted line.

Test

the_shared_preload_analyses_the_track_the_way_an_encode_does — the cache is
cleared, the preamble runs, the track must be in it. Removing the analysis call
turns it red with the message naming the symptom.

Gate

cargo fmt --all --check clean · cargo clippy --workspace --all-targets --features rustmotion/studio -D warnings clean · cargo test --workspace 1812 passed.