Reverts the sustain-relative modulation that made F.ENV SUS and the
cutoff knob feel dead while a note was held. The envelope now modulates
up from the base cutoff; at env amount 0 the cutoff equals the knob, and
lowering env amount darkens the held tone as expected.
Also extends the filter to 24/48 dB modes (extra TPT stages), adds the
matching combobox options, gives knobs a real blurred drop shadow, and
updates the AGENTS.md filterType choice count.