feat!: Protocol v2 client rewrite with Dongle/Session model

The full 1.0 client surface, matching the protocol v2 wire definitions from donglora-protocol 1.0.0.

* Public API is now async. The blocking Client<T: Transport> is replaced by Dongle (from donglora_client::connect / connect_with / ConnectOptions).

* src/client.rs and src/codec.rs are removed in favor of src/dongle.rs (high-level surface), src/session.rs (per-client tag/slot state), src/retry.rs (RetryPolicy + tx_with_retry), src/errors.rs (thiserror-based ClientError / ClientResult). src/protocol.rs is replaced by direct donglora-protocol imports.

* Transport is now tokio-serial for USB and tokio::net::{UnixStream, TcpStream} for mux, replacing the blocking serial + shim model.

* Dongle::tx_with_retry(data, &RetryPolicy) implements PROTOCOL.md §6.10 randomized backoff for CHANNEL_BUSY / EBUSY, returning a TxOutcome with per-attempt trace.

* Background keepalive task pings every 500 ms on host idle, tracking the 1 s inactivity timeout from §3.4 with 2x margin. Disable via ConnectOptions::keepalive(false).

* Auto-recovery from ERR(ENOTCONFIGURED): the Dongle caches the last SET_CONFIG and silently re-applies on session drop, retrying once.

* Error taxonomy switches from anyhow to thiserror. Transport-level conditions (Timeout, TransportClosed, ReaderExited, BadFrame) are first-class.

* Examples rewritten for the async API: examples/ping.rs, examples/tx.rs, examples/rx.rs all target the MeshCore US preset (910.525 MHz / BW 62.5 / SF 7 / CR 4/5 / sync 0x1424 / 20 dBm). examples/common/mod.rs holds the shared helper.

* tests/vectors.rs gains protocol-level roundtrip coverage anchored to the published donglora-protocol 1.0.0 types.

tests/vectors.rs

@@ -0,0 +1,98 @@
+//! Smoke-level vector round-trips through the `donglora-client` re-exports.
+//!
+//! The authoritative Appendix C vectors live in
+//! `donglora-protocol/tests/vectors.rs`. Here we check a representative
+//! subset just to confirm the client crate's re-exports cover the shapes
+//! a real application would use (so a refactor that accidentally hides
+//! one of them fails here rather than in downstream code).
+
+#![allow(clippy::unwrap_used, clippy::expect_used, clippy::panic)]
+
+use donglora_client::{
+    Command, LoRaBandwidth, LoRaCodingRate, LoRaConfig, LoRaHeaderMode, Modulation, RxOrigin, RxPayload, TxDonePayload,
+    TxResult,
+};
+use donglora_protocol::{FrameDecoder, FrameResult, MAX_WIRE_FRAME, encode_frame};
+
+fn meshcore_us() -> Modulation {
+    Modulation::LoRa(LoRaConfig {
+        freq_hz: 910_525_000,
+        sf: 7,
+        bw: LoRaBandwidth::Khz62,
+        cr: LoRaCodingRate::Cr4_5,
+        preamble_len: 16,
+        sync_word: 0x1424,
+        tx_power_dbm: 20,
+        header_mode: LoRaHeaderMode::Explicit,
+        payload_crc: true,
+        iq_invert: false,
+    })
+}
+
+/// `SET_CONFIG(MeshCore US)` should encode/decode byte-for-byte through
+/// the public API.
+#[test]
+fn set_config_roundtrip() {
+    let cmd = Command::SetConfig(meshcore_us());
+    let mut payload = [0u8; 256];
+    let n = cmd.encode_payload(&mut payload).unwrap();
+    let mut wire = [0u8; MAX_WIRE_FRAME];
+    let wire_n = encode_frame(cmd.type_id(), 42, &payload[..n], &mut wire).unwrap();
+
+    let mut decoder = FrameDecoder::new();
+    let mut saw: Option<(u8, u16, Vec<u8>)> = None;
+    decoder.feed(&wire[..wire_n], |res| match res {
+        FrameResult::Ok { type_id, tag, payload } => {
+            saw = Some((type_id, tag, payload.to_vec()));
+        }
+        FrameResult::Err(e) => panic!("decode error: {e:?}"),
+    });
+    let (type_id, tag, payload) = saw.expect("frame");
+    assert_eq!(type_id, cmd.type_id());
+    assert_eq!(tag, 42);
+
+    let parsed = Command::parse(type_id, &payload).unwrap();
+    match parsed {
+        Command::SetConfig(Modulation::LoRa(c)) => {
+            assert_eq!(c.freq_hz, 910_525_000);
+            assert_eq!(c.sync_word, 0x1424);
+            assert_eq!(c.tx_power_dbm, 20);
+        }
+        _ => panic!("expected LoRa SET_CONFIG"),
+    }
+}
+
+/// `TX_DONE` round-trips with airtime preserved.
+#[test]
+fn tx_done_roundtrip() {
+    let td = TxDonePayload { result: TxResult::Transmitted, airtime_us: 98_765 };
+    let mut buf = [0u8; TxDonePayload::WIRE_SIZE];
+    td.encode(&mut buf).unwrap();
+    let got = TxDonePayload::decode(&buf).unwrap();
+    assert_eq!(got, td);
+}
+
+/// `RX` event round-trips with metadata preserved.
+#[test]
+fn rx_roundtrip() {
+    let mut data = heapless::Vec::<u8, { donglora_protocol::MAX_OTA_PAYLOAD }>::new();
+    data.extend_from_slice(b"hello, lora!").unwrap();
+    let rx = RxPayload {
+        rssi_tenths_dbm: -812,
+        snr_tenths_db: 73,
+        freq_err_hz: -211,
+        timestamp_us: 1_000_000_000_000,
+        crc_valid: true,
+        packets_dropped: 3,
+        origin: RxOrigin::Ota,
+        data,
+    };
+    let mut buf = [0u8; RxPayload::METADATA_SIZE + 64];
+    let n = rx.encode(&mut buf).unwrap();
+    let got = RxPayload::decode(&buf[..n]).unwrap();
+    assert_eq!(got.rssi_tenths_dbm, -812);
+    assert_eq!(got.snr_tenths_db, 73);
+    assert_eq!(got.freq_err_hz, -211);
+    assert_eq!(got.packets_dropped, 3);
+    assert_eq!(&got.data[..], b"hello, lora!");
+}