SoundTouchOptions.interpolationStrategy carried `@defaultValue 'linear'`,
but the default is `lanczos`: RateTransposer's constructor ends with
`setInterpolationStrategy(interpolationStrategy ?? 'lanczos')`, and lanczos
is the only strategy registered out of the box.

The wrong value is worse than cosmetic here. `linear` ships in
@soundtouchjs/interpolation-strategy-linear and has to be registered before
use, so following the doc and passing `'linear'` throws "Unknown
interpolation strategy id" from construction. The README already describes
the registry correctly and shows the registerLinearStrategy opt-in, so this
was the only place stating otherwise.

Adds a spec pinning the default, which nothing covered — the existing
strategy specs all pass an explicit id.