Variable REVERB_LOWCUT_RANGEConst

The reverb's LOW cut, as an instance of REVERB_CUT_RANGE — same bounds, its own come-up value. 0 reads as "none" (mix_reverb.h), which is also DEFAULT_REVERB_STATE's seed, so a fresh reverb's low cut and its declared default are the same fact stated once.

Split from REVERB_CUT_RANGE rather than adding default there: the low and high cut share ONE travel (the C imposes one constraint on both) but do NOT share one come-up value, and a default on the shared range could only be honest for one of the two fields that read it.