Web Audio convolution reverb + stereo sample playback

Building on the new stereo render, two more Web Audio capabilities land - and both are node classes real pages use that previously did nothing. ConvolverNode is the reverb/impulse-response node: it convolves a signal against an impulse response buffer (a room, a hall, a speaker cabinet) to make it sound like it was played there. It is implemented as exact FIR convolution in a fixed f64 order, so it is deterministic by construction like the rest of the audio engine and its math is checked against known cases: a single-tap unit impulse is the exact identity, an impulse shifted by two samples is exactly a two-sample delay, and a two-tap averaging response gives the exact FIR output. AudioBufferSourceNode also plays STEREO buffers now, not just channel zero - a two-channel sample plays each channel to its own side. Media moves 37 to 38; overall holds at 51.