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.

src/crc.rs

@@ -0,0 +1,57 @@
+//! CRC-16/CCITT-FALSE — the wire-level integrity check used by DongLoRa Protocol.
+//!
+//! Polynomial `0x1021`, initial value `0xFFFF`, no reflection, XOR-out `0x0000`.
+//! This is the same variant also known as CRC-16/AUTOSAR or
+//! CRC-16/IBM-3740. It is **NOT** the same as CRC-16/KERMIT,
+//! CRC-16/XMODEM, or plain "CRC-16/CCITT" — those use different initial
+//! values or reflect their inputs.
+//!
+//! Check value: `crc16("123456789") == 0x29B1`. A module-level test pins
+//! this, and `tests/vectors.rs` anchors every Appendix C wire example.
+
+/// CRC-16/CCITT-FALSE of `data`.
+pub fn crc16(data: &[u8]) -> u16 {
+    let mut crc: u16 = 0xFFFF;
+    for &byte in data {
+        crc ^= (byte as u16) << 8;
+        for _ in 0..8 {
+            if crc & 0x8000 != 0 {
+                crc = (crc << 1) ^ 0x1021;
+            } else {
+                crc <<= 1;
+            }
+        }
+    }
+    crc
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn check_value_matches_spec() {
+        // PROTOCOL.md §2.2: CRC-16/CCITT-FALSE of ASCII "123456789" is 0x29B1.
+        assert_eq!(crc16(b"123456789"), 0x29B1);
+    }
+
+    #[test]
+    fn empty_input_is_initial_value() {
+        // No reflection, no XOR-out: empty input leaves the init value untouched.
+        assert_eq!(crc16(&[]), 0xFFFF);
+    }
+
+    #[test]
+    fn single_zero_byte() {
+        // 0xFFFF XOR 0x0000 = 0xFFFF, then 8 shift-and-maybe-xor rounds.
+        // Worked out by hand against the reference impl in Appendix B.
+        assert_eq!(crc16(&[0x00]), 0xE1F0);
+    }
+
+    #[test]
+    fn differs_from_kermit_variant() {
+        // CRC-16/KERMIT of "123456789" is 0x2189 — if we ever accidentally
+        // swap init or reflection, this vector catches it.
+        assert_ne!(crc16(b"123456789"), 0x2189);
+    }
+}