Doubling a track's speed live can go two ways: the turntable way, where
everything jumps up an octave, or the pitch-preserving way, where the
groove speeds up but stays at the same pitch. This page plays both, and
each tier leaves an assertable signature proving which path ran.
playbackRate tier: applyLiveDoubleSpeed() crossfades to a
replacement source whose playbackRate.value must be exactly 2.0,
stopping the old source after the 50 ms fade (the audible octave jump).
Resample tier (preservePitch: true): the returned buffer
keeps the ORIGINAL length, newLoopLength === floor(len/2), and
the trailing half must be exactly silent — 2× resampling writes the
compressed audio into the first half only; that silence is the tier's
assertable signature. resetLiveSpeed() restores rate 1.0.
Dependencies are injected explicitly (no window bus). Resample + error
contract asserts spot-run headless in Node pre-ship.
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