[refactor](trx-rs): resolve all improvement areas (P0-P3)

Addresses every item in docs/Improvement-Areas.md:

P0 - Plugin signing: new src/trx-app/src/plugins.rs with SHA-256 checksum
     manifest, filename allowlisting, API version compatibility checks,
     and cross-platform file permission validation.

P1 - Session store mutex poisoning: all .unwrap() calls on RwLock/Mutex in
     auth.rs replaced with .unwrap_or_else(|e| e.into_inner()) + warning logs.
   - TCP listener rate limiting: added ConnectionTracker with per-IP connection
     cap (10 concurrent connections per IP).
   - RigState refactoring: decoder fields grouped into DecoderConfig and
     DecoderResetSeqs sub-structs with #[serde(flatten)] for wire compat.
   - spawn_blocking timeout: satellite pass computation wrapped in 30s timeout.

P2 - Command handler macro: rig_command! macro generates 7 unit-struct command
     implementations, reducing ~200 lines of boilerplate.
   - Protocol versioning: added protocol_version field to ClientEnvelope and
     ClientResponse; improved unknown command error handling in parse_envelope.
   - Unsafe string: replaced from_utf8_unchecked with safe from_utf8().expect().
   - Dead code: removed 2 unnecessary annotations, documented remaining 4.

P3 - Tests: added 4 unit tests for history_store.rs (round-trip, expiry, etc).
   - FT-817 VFO: improved inference for ambiguous same-frequency case.
   - Configurator: implemented serial port detection via tokio_serial.
   - Plugin versioning: integrated into plugin manifest (api_version field).
   - Naming: documented as intentional semantic distinctions, not inconsistencies.

https://claude.ai/code/session_01Gj1vEkP6GKVcVaMqzFW885
Signed-off-by: Claude <noreply@anthropic.com>

Cargo.lock

@@ -2428,6 +2428,17 @@ dependencies = [
  "digest",
 ]
 
+[[package]]
+name = "sha2"
+version = "0.10.9"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "a7507d819769d01a365ab707794a4084392c824f54a7a6a7862f8c3d0892b283"
+dependencies = [
+ "cfg-if",
+ "cpufeatures",
+ "digest",
+]
+
 [[package]]
 name = "sharded-slab"
 version = "0.1.7"
@@ -2964,7 +2975,10 @@ name = "trx-app"
 version = "0.1.0"
 dependencies = [
  "dirs",
+ "hex",
+ "libc",
  "serde",
+ "sha2",
  "thiserror 2.0.17",
  "toml",
  "tracing",
@@ -3063,6 +3077,7 @@ dependencies = [
  "clap",
  "dialoguer",
  "tempfile",
+ "tokio-serial",
  "toml_edit 0.22.27",
 ]