[feat](trx-server): add JSON TCP listener for client connections

Add a JSON-over-TCP listener so trx-client can connect to trx-server.
Speaks the ClientEnvelope/ClientResponse protocol from trx-core::client.

- New listener.rs module with per-client connection handling
- ListenConfig/AuthConfig in config.rs (default: 127.0.0.1:4532)
- CLI args --listen and --port for override
- Optional token-based authentication
- Updated example config with [listen] section

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Signed-off-by: Stanislaw Grams <stanislawgrams@gmail.com>

src/trx-server/Cargo.toml

@@ -11,6 +11,7 @@ edition = "2021"
 tokio = { workspace = true, features = ["full"] }
 tokio-serial = { workspace = true }
 serde = { workspace = true, features = ["derive"] }
+serde_json = { workspace = true }
 toml = { workspace = true }
 tracing = { workspace = true }
 tracing-subscriber = { workspace = true }