perf(viz): snappier reactivity across braille visualisers

[vxcozy]

Summary

• Retune SpectrumScaler AGC (attack 50ms → 25ms, release 2.5s → 1.2s) so the bar family snaps to transients and tracks song sections instead of averaging five seconds of audio into the ceiling.
• Tighten the shared EnergyTracker release coefficient 0.88 → 0.75 (tau 258ms → 115ms) on 16 viz whose energy drives amplitude, brightness, or spawn rate.
• Pull bars_outline per-bar release into line with bars_dot (0.85 → 0.80) so outline bars decay like their filled cousins.

Why

Users reported the entire visualiser suite feels sluggish, not just the bar family. Two compounding sources:

1. PR fix(viz): log/dB scaling for bar visualisers #57's AGC was tuned for safety. A 2.5s release ceiling averages loudness over a whole song section; during that window bars lag behind the current dynamics. Tightening to 1.2s lets the ceiling track choruses/verses without strobing on brief dips. Attack also dropped to 25ms so a kick drum is reflected the same frame it arrives.

2. EnergyTracker default pattern (0.5, 0.88, 500.0) is used by ~20 viz. The 0.88 retention on release means brightness and amplitude carry forward ~260ms of history — perceptually "moving on a delay". 0.75 retention halves that to ~115ms, which is still enough to hide FFT jitter but not enough to read as lag.

The attack side (0.5 retention, tau ~48ms) was already fine and left alone. Transient-detection viz (pulse, firework at 0.3/0.85) deliberately use a slow baseline so their energy-delta triggers work — left untouched. Texture viz (tunnel, vortex, plasma, metaballs, moire at ≥0.9 retention) drive smoothly-evolving shaders where "snappy" reads as jitter — left untouched.

Per-viz changes

Viz	Parameter	Before → After	Rationale
scaling.rs	SpectrumScaler ATTACK	0.48 → 0.735	50ms → 25ms tau; kick transients snap same-frame
scaling.rs	SpectrumScaler RELEASE	0.013 → 0.027	2.5s → 1.2s tau; ceiling tracks song sections
bars_dot.rs	EnergyTracker release	0.88 → 0.75	overlay brightness tracks the beat
bars_outline.rs	EnergyTracker release	0.88 → 0.75	overlay brightness tracks the beat
bars_outline.rs	per-bar release blend	0.85 → 0.80	match bars_dot; bars stop hanging above the music
classic_peak.rs	EnergyTracker release	0.88 → 0.75	peak-cap accent tracks dynamics
heartbeat.rs	EnergyTracker release	0.88 → 0.75	ECG amplitude follows beat envelope
wave.rs	EnergyTracker release	0.88 → 0.75	trace brightness dims between transients
scope.rs	EnergyTracker release	0.88 → 0.75	phosphor responds within ~100ms
matrix.rs	EnergyTracker release	0.88 → 0.75	drop speed drops on phrase boundaries
terrain.rs	EnergyTracker release	0.88 → 0.75	mountain heights track current loudness
retro.rs	EnergyTracker release	0.88 → 0.75	sun radius pulses, not glides
rain.rs	EnergyTracker release	0.88 → 0.75	spawn probability tracks the beat
binary.rs	EnergyTracker release	0.88 → 0.75	bit field reshuffles with phrases
butterfly.rs	EnergyTracker release	0.88 → 0.75	wings flap with the beat
scatter.rs	EnergyTracker release	0.88 → 0.75	scatter density follows the beat
fire.rs	EnergyTracker release	0.88 → 0.75	heat injection pulses with the music
sakura.rs	EnergyTracker release	0.88 → 0.75	petal emission breathes with the beat
ripples.rs	EnergyTracker release	0.88 → 0.75	drop-spawn gating reacts to current beat

Deliberately untouched: pulse, firework (transient detection needs slow baseline), tunnel, vortex, plasma, metaballs, moire (texture viz — jitter would read as noise, not snap).

Test plan

• cargo fmt --check
• cargo clippy --workspace --all-targets -- -D warnings
• cargo test --workspace --all-features (all existing scaling:: + per-viz tests pass under new coefficients)
• manual: play radio, cycle through braille viz, confirm each "moves with the music" instead of lagging a quarter second
• manual: confirm pulse/firework transient triggers still fire on beat drops
• manual: confirm texture viz (plasma, metaballs, moire) still look smooth, not jittery

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