[feat](trx-server): implement multi-rig support
Enable N simultaneous rig backends in one server process:
- rig_handle.rs: new RigHandle { rig_id, rig_tx, state_rx } thin struct
- config.rs: RigInstanceConfig, rigs: Vec<RigInstanceConfig> in ServerConfig,
resolved_rigs() (synthesises legacy flat fields as id="default"),
validate() checks unique rig IDs and audio ports; MR-08 config tests
- audio.rs: replace four OnceLock<Mutex<VecDeque>> statics with
DecoderHistories { aprs, ft8, wspr } Arc struct; decoder/listener
functions now take Arc<DecoderHistories> for per-rig isolation
- rig_task.rs: add rig_id + histories: Arc<DecoderHistories> to
RigTaskConfig; clear_*_history calls use ctx.histories instance methods
- listener.rs: run_listener takes Arc<HashMap<String, RigHandle>> +
default_rig_id; routes envelope.rig_id to correct rig; GetRigs fast
path aggregates all rig states; all responses include rig_id field
- main.rs: loop over resolved_rigs(); spawn_rig_audio_stack() helper;
builds Arc<HashMap<String, RigHandle>> passed to run_listener
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Signed-off-by: Stan Grams <sjg@haxx.space>
sjg
0b8c408c17