[feat](trx-backend): set capability flags; add SDR filter state; add UC-08 tests

Set tx/tx_limit/vfo_switch/filter_controls/signal_meter on all backends:
- FT-817, FT-450D, dummy: tx=true, tx_limit=true, vfo_switch=true,
  filter_controls=false, signal_meter=true
- SoapySDR: tx=false, tx_limit=false, vfo_switch=false,
  filter_controls=true, signal_meter=true

SoapySDR backend now stores bandwidth_hz and fir_taps fields; overrides
set_bandwidth, set_fir_taps, and filter_state on RigCat to expose live
DSP state in the snapshot.

Add UC-08 unit tests on dummy backend asserting tx capabilities present
and filter_controls absent, and that filter_state returns None.

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-ft817/src/lib.rs

@@ -207,6 +207,11 @@ impl Ft817 {
                 rpt: false,
                 split: false,
                 lock: true,
+                tx: true,
+                tx_limit: true,
+                vfo_switch: true,
+                filter_controls: false,
+                signal_meter: true,
             },
             access: RigAccessMethod::Serial {
                 path: path.to_string(),