7/29: I printed up a small jig to test pedal sensors. It lets me turn a magnet by hand over the sensor; this helped me pinpoint a not-bug in the position sensor coding. It turns out the AS5055A output is 0-1FFF (13 bits) over 360 degrees; the datasheet must be referring to 12 bits over 180 degrees. Anyhoo, the position sensor is tracking fine now.
7/30: The last bug has been obliterated, all scope waveforms track on all effects - time for the acid test! I hooked up a guitar to the input, and the output to the Aux In jack of a cheapo countertop stereo, and ... a split-second of sound, and <pop> no audio. Power off/on, a split -second of sound, and <pop>. WTF?
I hooked in the oscillator I was testing with, everything's back to normal; at least nothing blew. 'Sample/Hold' and 'Theremin' sound totally *awesome* as you move the magnet! Most of the rest work as expected, but Chorus seems a bit odd; then again, it could be that I'm getting cancelling effects from a single-frequency input tone.
After that, I tried the guitar again - NOW it decides to behave! Maybe I'm sitting too close to the speaker, and feedback tripped something.
All in all, it's working, just not reliably. More testing to come.
8/6: Came across a description of something called 'Track & Hold' in an essay on sample & hold circuits. It is now effect number 16 on the device. Given that the selector is a 12-position encoder, that's 1-1/3 turns of the knob to scroll through the list. I'm declaring a moratorium on new effects until I get some opinions on what to cull. My Carpal Tunnels will thank me later. Plus, as Brian mentioned in the review, I need to focus on the mechanical side of things.
8/11: My first print of a foot pedal mechanism came out with a weak layer through everything; the plastic is pale, with visible gaps between layers. Pieces fall off at the merest touch. Still, it gives me some insight into what might and might not work.