dsp: a bend knob on guitar and analog

[mishan]

Pitch bends for leads. Stacked on #333 because it touches analog.dsp.

A slewed @bend knob, in semitones, on guitar.dsp (±2) and analog.dsp (±12). It is channel-wide, like a pitch wheel, which is how a monophonic lead is bent on real instruments too. A piece rides it with a chanarg sink:

chain bends {
    stage s gen::steps { values = "0 0 1 1"; period = 1 beats; min = 0; max = 2; };
    sink { instrument = lead; chanarg = "bend"; };
};

On the guitar the bend moves the comb's length but not its decay.

A per-note bend (MPE-style) would need a new event kind through the scheduler, the worklet protocol and MIDI out. This PR doesn't attempt that.

Testing

• ctest -R "dsp|fx|catalog|gencheck" passes.
• At bend = 0 the pitch is multiplied by exactly 1: gen/mirrorball.gen (guitar) renders byte-identical to before.
• A held A4 with the row above measures 440.1 Hz, then 493.9 Hz (B4, +2 semitones) after the step.