feat!: v0.2.0 — ping-on-connect validation, try_connect, publish polish

- Add ping-on-connect handshake: all connect functions validate the device
  within 200ms, rejecting non-DongLoRa serial devices immediately
- Add try_connect() — non-blocking connect that errors instead of blocking
  forever when no USB device is present
- Add payload size validation in transmit() (max 256 bytes)
- Add ErrorCode: impl std::error::Error
- Fix drain_rx error handling (no longer swallows original error)
- Fix repository URL to github.com/donglora/client-rs
- Remove wire-level tag constants from top-level re-exports
- Add rust-version (MSRV 1.85), homepage, exclude fields to Cargo.toml
- Add examples/ping.rs
- Allow clippy unwrap/expect in test modules

Cargo.lock

@@ -44,7 +44,7 @@ checksum = "773648b94d0e5d620f64f280777445740e61fe701025087ec8b57f45c791888b"
 
 [[package]]
 name = "donglora-client"
-version = "0.1.0"
+version = "0.2.0"
 dependencies = [
  "anyhow",
  "serialport",