feat!: protocol v2 wire format and module restructure

The wire protocol is re-specified as DongLoRa Protocol v2 (see PROTOCOL.md). Every type_id, tag-correlation rule, and field layout is incompatible with 0.1.0.

* Add PROTOCOL.md as the normative spec for the v2 wire format.

* Replace framing.rs with a streaming FrameDecoder that emits (type_id, tag, payload) triples on each COBS-delimited, CRC-checked frame.

* Split the single-file lib.rs into dedicated modules: frame, commands, events, errors, modulation, info, crc, chip_id. Re-exports preserve convenience imports at the crate root.

* Replace RadioConfig / Bandwidth with a Modulation enum covering LoRa, FSK/GFSK, LR-FHSS, and FLRC, each with its own typed parameter struct.

* Add TxFlags, RxOrigin, SetConfigResult, OkPayload to model v2 response shapes, and RadioChipId for GET_INFO.

* Rewrite fuzz corpus: replace the 4 cobs_* / command_roundtrip targets with decoder_feed, frame_roundtrip, lora_config_roundtrip, and parse_device_message; drop parse_radio_config.

* Expand proptest and mutation-killer coverage for the new wire shape.

* Rewrite canonical-byte test vectors against the v2 spec.

fuzz/fuzz_targets/parse_command.rs

@@ -1,14 +1,20 @@
-//! `Command::from_bytes` must never panic on arbitrary input.
-//!
-//! Any accepted command must also respect the heapless payload bound.
+//! `Command::parse` must never panic on arbitrary `(type_id, payload)`
+//! input. Accepted commands must respect the OTA payload bound.
 
 #![no_main]
 
-use donglora_protocol::{Command, MAX_PAYLOAD};
+use arbitrary::Arbitrary;
+use donglora_protocol::{Command, MAX_OTA_PAYLOAD};
 use libfuzzer_sys::fuzz_target;
 
-fuzz_target!(|data: &[u8]| {
-    if let Some(Command::Transmit { payload, .. }) = Command::from_bytes(data) {
-        assert!(payload.len() <= MAX_PAYLOAD);
+#[derive(Debug, Arbitrary)]
+struct Input<'a> {
+    type_id: u8,
+    payload: &'a [u8],
+}
+
+fuzz_target!(|input: Input| {
+    if let Ok(Command::Tx { data, .. }) = Command::parse(input.type_id, input.payload) {
+        assert!(data.len() <= MAX_OTA_PAYLOAD);
     }
 });