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/Cargo.toml

@@ -22,6 +22,13 @@ features = ["std"]
 [workspace]
 members = ["."]
 
+[[bin]]
+name = "decoder_feed"
+path = "fuzz_targets/decoder_feed.rs"
+test = false
+doc = false
+bench = false
+
 [[bin]]
 name = "parse_command"
 path = "fuzz_targets/parse_command.rs"
@@ -30,43 +37,22 @@ doc = false
 bench = false
 
 [[bin]]
-name = "parse_radio_config"
-path = "fuzz_targets/parse_radio_config.rs"
+name = "parse_device_message"
+path = "fuzz_targets/parse_device_message.rs"
 test = false
 doc = false
 bench = false
 
 [[bin]]
-name = "cobs_decoder_feed"
-path = "fuzz_targets/cobs_decoder_feed.rs"
+name = "frame_roundtrip"
+path = "fuzz_targets/frame_roundtrip.rs"
 test = false
 doc = false
 bench = false
 
 [[bin]]
-name = "cobs_roundtrip"
-path = "fuzz_targets/cobs_roundtrip.rs"
-test = false
-doc = false
-bench = false
-
-[[bin]]
-name = "command_roundtrip"
-path = "fuzz_targets/command_roundtrip.rs"
-test = false
-doc = false
-bench = false
-
-[[bin]]
-name = "cobs_decoder_multi_frame"
-path = "fuzz_targets/cobs_decoder_multi_frame.rs"
-test = false
-doc = false
-bench = false
-
-[[bin]]
-name = "cobs_chunk_invariant"
-path = "fuzz_targets/cobs_chunk_invariant.rs"
+name = "lora_config_roundtrip"
+path = "fuzz_targets/lora_config_roundtrip.rs"
 test = false
 doc = false
 bench = false