The first base32 symbol carries 3 data bits (26 symbols = 130 bits, 2 spare),
but decode only accepted values 0-3, so ~half of generated keys failed to parse.
Adds an edge-case test, rustfmt config, Cargo.lock, and on-demand cross targets
instead of installing all seven in rust-toolchain.toml.
Co-Authored-By: Claude Opus 5.5 <[email protected]>