need special envelope for punch in/out

[catfact]

currently when setting rec=0 and pre=1 at the same time, we get a dip in volume in the buffer. i think this is because both levels are passed through a 1-pole LPF, acquiring exp/log envelopes.

punch in/out probably requires a tuned pair of envelopes similar to the pre/rec xfade envelopes.

[catfact]

using small slew times sounds fine actually. the tradeoffs for a dedicated envelope seem excessive (e.g. having to deal with moving targets during an envelope segment.) closing this for now.