[don't merge] zap gremlins on SVF input, clamp FC

[catfact]

seems to fix reported noise issues on recent build.

not ready to merge (and maybe not really the correct approach; e.g. gshould sanitize earlier);

at minimum needs benchmarking and probably optimization pass to offset added overhead

[catfact]

FWIW, the approach SuperCollider takes is to apply a similar (better) zapgremlins function to filter I/O, but only once at the end of each processing block. thus preventing gremlins(/denorms) from recirculating, with minimal steady-state CPU cost.

but if possible it would be nice if we could know that all signals on internal busses are always safe.

[catfact]

found the underlying source of bad values instead