Function append_with_trims

Shared append tail for both the PCM and streaming decode paths: apply trim (and optional take/loop) then route through the DSP chain into the sink.

(
    sink: &SpatialSink,
    source: S,
    dsp: Arc<DspControl>,
    trim_start: Duration,
    play_duration: Option<Duration>,
    looped: bool,
)