[feat](trx-protocol): create protocol unification crate

Add new crate to centralize protocol conversion logic:
- codec module: mode parsing/formatting, envelope parsing
- auth module: token validation, bearer prefix handling
- mapping module: ClientCommand <-> RigCommand conversion

Includes 76 comprehensive tests covering all command variants,
error cases, and round-trip conversions. Removes duplication
across listener, remote_client, and HTTP-JSON frontend.

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

src/trx-protocol/Cargo.toml

@@ -0,0 +1,13 @@
+# SPDX-FileCopyrightText: 2025 Stanislaw Grams <stanislawgrams@gmail.com>
+#
+# SPDX-License-Identifier: BSD-2-Clause
+
+[package]
+name = "trx-protocol"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]
+serde = { workspace = true, features = ["derive"] }
+serde_json = { workspace = true }
+trx-core = { path = "../trx-core" }