feat: import wire protocol types from firmware

Copied src/protocol.rs from firmware verbatim, added crate-level #![no_std],
migrated the defmt::Format derive gate from cfg_attr(not(test), ...) to
cfg_attr(feature = "defmt", ...) so host-side (no-default-features) builds
compile cleanly. Tests unchanged; all 35 pass.

Firmware still owns the file for now — the re-export swap lands in a
follow-up firmware commit to keep each change standalone.

.gitignore

@@ -0,0 +1,5 @@
+/target
+/fuzz/target
+/fuzz/corpus
+/fuzz/artifacts
+Cargo.lock