Implement `wait gpio` for idx >= 32

An instruction like `wait 0 gpio N` refers to the absolute GPIO number. sdk pioasm tracks which numbers have been used in 16-bit blocks, reporting an error if pins in the "low" and "high" ranges are both used. This is a bit funky and it's not clear what the behavior should be. sdk pioasm gives some weird results, which I reported as an issue: https://github.com/raspberrypi/pico-sdk/issues/2141

Start by tracing how the assembler handles `wait 0 gpio N`, especially GPIO numbers at or above 32. Read the linked pico-sdk issue 2141 and compare its behavior with this assembler. Before coding, establish the intended low/high-range semantics; done means the behavior is defined and covered for idx >= 32.

Written by the indexing model from the issue text.