[feat](trx-backend): VirtualChannelManager trait + SdrVirtualChannelManager impl

Add VirtualChannelManager trait in trx-core::vchan with types VChannelInfo,
VChanError, and SharedVChanManager alias. Re-export from trx-backend::vchan.

Implement SdrVirtualChannelManager in trx-backend-soapysdr:
- Wraps Arc<SdrPipeline> + shared AtomicI64 center_hz
- add_channel / remove_channel / set_channel_freq / set_channel_mode
- Slot-stability: on remove, shifts pipeline_slot for surviving channels
- update_center_hz: recomputes IF offsets for all virtual channels on retune
- update_primary_meta: keeps channel-0 freq/mode in sync for API consumers

Wire into SoapySdrRig (holds Arc<SdrVirtualChannelManager>, exposes
channel_manager()), SdrPipeline (shared_center_hz AtomicI64), and RigHandle
(vchan_manager: Option<SharedVChanManager>). main.rs extracts the manager
before boxing the SDR rig and stores it in the handle.

Add max_virtual_channels to SdrConfig (default 4, TOML-configurable).
Add 5 unit tests: add, remove, permanent guard, cap, out-of-bandwidth.

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

src/trx-server/trx-backend/trx-backend-soapysdr/Cargo.toml

@@ -13,6 +13,7 @@ trx-core = { path = "../../../trx-core" }
 trx-rds = { path = "../../../decoders/trx-rds" }
 tokio = { workspace = true, features = ["sync", "rt"] }
 serde = { workspace = true }
+uuid = { workspace = true }
 tracing = { workspace = true }
 num-complex = "0.4"
 rustfft = "6"