Three time-based throttles used by the audio-progress handler move into
`src/store/playbackThrottles.ts`:
- **Live progress emit** (1.5 s / 0.9 s position delta) — feeds the
high-frequency pub/sub
- **Store progress commit** (20 s / 5 s position delta) — writes the
Zustand store
- **Normalization UI update** (120 ms) — live dB readout throttle
Module owns three private mutables, exports five constants (the four
window/delta gates + NORMALIZATION_UI_THROTTLE_MS) and six accessors
(get / mark for each), plus `resetProgressEmitThrottles` for the
track-boundary reset path that handleAudioPlaying calls.
playerStore's six direct-access sites + two reset writes collapse to
imports. Behaviour preserved verbatim.
13 focused tests pin each throttle's get/mark cycle, the partial reset
(progress only, not normalization), and constant values.
playerStore +6 LOC net (more import surface than freed mutables) —
shrinkage will come back on the next bigger encapsulation; this PR's
win is logical separation + testability rather than line count.
Frank Stellmacher