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.

Cargo.toml

@@ -0,0 +1,35 @@
+[package]
+name = "donglora-protocol"
+version = "0.1.0"
+edition = "2024"
+rust-version = "1.85"
+authors = ["Stephen Waits <steve@waits.net>"]
+license = "MIT"
+description = "DongLoRa wire protocol types and COBS framing — shared between firmware and host crates"
+repository = "https://github.com/donglora/protocol"
+homepage = "https://github.com/donglora/protocol"
+keywords = ["lora", "radio", "protocol", "no-std"]
+categories = ["embedded", "no-std", "encoding"]
+exclude = ["fuzz", ".jj"]
+
+[dependencies]
+heapless = "0.9"
+ucobs = "0.3"
+defmt = { version = "1", optional = true }
+
+[features]
+default = ["defmt"]
+# Enables `defmt::Format` derives on public types. On by default so firmware
+# (the primary no_std consumer) just works; host crates disable default-features.
+defmt = ["dep:defmt"]
+# Enables `Debug` derives and other std-only niceties for host-side fuzz /
+# proptest builds. No-op on firmware.
+std = []
+
+[dev-dependencies]
+proptest = "1"
+
+[lints.clippy]
+unwrap_used = "deny"
+expect_used = "deny"
+panic = "deny"