Duck Spotify volume while speaking instead of talking over it

[malpern]

What

The extension already paused YouTube while VoxClaw spoke, but Spotify web playback was left at full volume, so speech competed with the music. This fades playing open.spotify.com tabs down to 20% for the duration of the speech, then restores each tab's original volume (300ms down, 500ms up). YouTube's pause/resume behavior is unchanged.

Notes

• Restore is held 1.5s (RESTORE_HOLD_MS). /status reports reading:false during the speech queue's 1s inter-item gap, so without the hold the volume yo-yos between queued utterances.
• Volume is also restored if the VoxClaw listener disappears mid-speech or polling is toggled off, so a tab is never left stuck quiet.
• Each tab's original volume is preserved per-tab; tabs already at or below the duck level are left alone.
• Ducking moves Spotify's own volume slider, since that slider reads element.volume — the dip is visible in the UI, not just audible.
• Tunables are the four constants at the top of service_worker.js.

Verification

• Fade math simulated against a stepped requestAnimationFrame clock: ducks to exactly 0.2, restores to the original 0.85, and skips elements already quieter than the duck level.
• Probed Spotify's real DOM via an instrumented build: the player is a media element in the main document (shallow=1, deep=1, readyState=4), so querySelectorAll("video, audio") reaches it with no shadow-DOM traversal.
• Installed unpacked in Dia (Chromium 150) and confirmed the service worker polls /status at a steady 1Hz against a running VoxClaw.
• Not exercised: the !media.paused gate, i.e. ducking during actual playback.

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