[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>

src/trx-client/trx-frontend/trx-frontend-rigctl/src/server.rs

@@ -656,14 +656,7 @@ mod tests {
             server_longitude: None,
             pskreporter_status: None,
             aprs_is_status: None,
-            aprs_decode_enabled: false,
-            hf_aprs_decode_enabled: false,
-            cw_decode_enabled: false,
-            ft8_decode_enabled: false,
-            ft4_decode_enabled: false,
-            ft2_decode_enabled: false,
-            wspr_decode_enabled: false,
-            lrpt_decode_enabled: false,
+            decoders: trx_core::DecoderConfig::default(),
             cw_auto: false,
             cw_wpm: 0,
             cw_tone_hz: 0,