Fix echo feedback pot index and convert CLI bugs

Addresses items 1, 3, 4, and 5 reported in #101:

1. audio/echo.h (echo_init): echo_effect.pots defines Delay, Depth, and Feedback at indices 0..2, but echo_init() was reading pot[3] instead of pot[2] for echo.feedback, leaving the Feedback knob ineffective on the pedal and CLI.
2. convert.c (describe): Write effect and pot descriptions to stderr instead of stdout so piping raw s32le audio to - (stdout) is not corrupted or misaligned by text output.
3. convert.c (modify_pots): Allow live --control updates for all 10 pots (idx <= 9 instead of idx <= 3) and wrap the case 'p': variable declarations in a block scope for standard C compliance.
4. convert.c (main):
   • Match short_name in addition to name when looking up an effect (fixing ./convert eq ... / make eq).
   • Skip effects that do not implement step and init (preventing a NULL dereference on ./convert usb ...).
   • Exit cleanly with an error message when no effect is selected instead of dereferencing eff == NULL in describe().