[chore](trx-rs): resolve all compiler and clippy warnings

- trx-server/rig_handle: remove dead vchan_manager field (was set but
  never read after the virtual-channel refactor)
- trx-server/listener: remove now-missing vchan_manager initializer
- trx-server/main: remove vchan_manager_for_handle intermediates that
  only fed the dropped field
- trx-server/audio: suppress too_many_arguments on run_audio_listener
- trx-frontend-http/server: suppress too_many_arguments on build_server
- trx-core/vchan: update module doc comment to not reference the
  removed RigHandle::vchan_manager field

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Signed-off-by: Stan Grams <sjg@haxx.space>

src/trx-core/src/vchan.rs

@@ -7,7 +7,7 @@
 //! A *virtual channel* is an independent DSP slice within the capture bandwidth
 //! of an SDR rig.  Each has its own frequency offset, demodulation mode, and
 //! PCM audio broadcast.  Traditional (non-SDR) rigs do not support virtual
-//! channels; their `RigHandle::vchan_manager` field will be `None`.
+//! channels; virtual channel operations are not available for traditional rigs.
 
 use std::sync::Arc;