Add stdio remotes, drop R/, log to stderr (v0.8.2)

[guyte149]

Three changes that bring the remote-syntax parser to chisel parity and make stdio: remotes actually usable:

• Add stdio:<host>:<port> (and stdio:<port>, defaulting remote host to 127.0.0.1) forward remotes. The client pipes its own stdin/stdout to/from the QUIC stream instead of binding a local listener; on stdin EOF the session shuts down cleanly. Server-side dispatch is unchanged. Reverse stdio (R:stdio:...) and stdio:socks are rejected at parse time. Wire model: new stdio: bool on RemoteRequest, serialized with #[serde(default)].

• Drop the Rusnel-specific R/ reverse-prefix shorthand. Chisel only accepts R: and we now match exactly. R/foo is now a parse error.

• Initialize tracing with with_writer(std::io::stderr) for all CLI modes. Required for stdio (stdout is the data plane); good practice for the rest.

Tests: 12 new parser unit tests for the stdio + R/ changes, plus a new tests/stdio.rs e2e smoke that spawns the rusnel binary as a subprocess with a stdio remote pointing at an in-process echo target, round-trips three framed messages through its stdin/stdout, and asserts the child exits 0 after stdin EOF.