[feat](trx-rs): configurable DIG sideband for SDR (auto/USB/LSB)

DIG has no inherent sideband; on the SDR backend it was always demodulated
as USB. Make it resolve to USB or LSB via a policy that defaults to the
amateur SSB/data convention (USB at/above 10 MHz, LSB below) and can be
overridden globally from the advanced radio controls or per-bookmark.

Design: the logical DIG mode is kept in RigState (display, decoder gating)
while the SDR pipeline is handed a concrete USB/LSB demodulator resolved from
(policy, dial frequency). Resolution happens at the boundary — the rig for
the primary channel and the virtual-channel manager for vchans — so the hot
DSP/demod path is untouched. The resolved sideband is only re-pushed when it
actually changes (e.g. tuning DIG/Auto across 10 MHz), keeping ordinary
tuning glitch-free.

Core/protocol:
- New `DigSidebandPolicy { Auto, Usb, Lsb }` with `resolve(freq)` and an
  `effective_demod_mode()` helper (trx-core), re-exported at the crate root.
- `RigCommand::SetSdrDigSideband`, `RigSdr::set_sdr_dig_sideband`, and a
  `RigFilterState.sdr_dig_sideband` field for state sync; wired through the
  ClientCommand mapping.

Config: `[rig.sdr] dig_sideband = "auto"` (regenerated trx-rs.toml.example).

SDR backend: the vchan manager owns the shared policy (atomic); the rig
applies it to the primary channel and, on `set_sdr_dig_sideband`, re-resolves
all DIG virtual channels.

Frontend: a mode-gated "DIG sideband" selector in the SDR advanced controls
(POST /set_sdr_dig_sideband), reflecting server state; bookmarks gain an
optional `dig_sideband` field (form selector shown only for DIG) that, on
apply, sets the global policy before switching to DIG. The scheduler honours
it for automated bookmark activation too.

Tests: policy resolution / effective-mode / u8+parse round-trips (trx-core);
a vchan integration test asserting a DIG channel resolves to LSB below
10 MHz, flips with the policy, and still lists as DIG.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UiK871ht2uPFBHtMbxy3wD
Signed-off-by: Stan Grams <sjg@haxx.space>

src/trx-client/trx-frontend/trx-frontend-http/assets/web/index.html

@@ -385,6 +385,14 @@ SPDX-License-Identifier: GPL-2.0-or-later
                   </label>
                   <button id="sdr-nb-threshold-set" type="button" class="wfm-inline-btn">Set</button>
                 </div>
+                <label class="wfm-control" id="sdr-dig-sideband-wrap" style="display:none;">
+                  <span class="wfm-control-label" title="Sideband used to demodulate DIG. Auto = USB &ge; 10 MHz, LSB below.">DIG sideband</span>
+                  <select id="sdr-dig-sideband" class="status-input">
+                    <option value="auto">Auto (by band)</option>
+                    <option value="usb">USB</option>
+                    <option value="lsb">LSB</option>
+                  </select>
+                </label>
               </div>
             </div>
             <div class="full-row label-below-row" id="vchan-row">
@@ -721,6 +729,14 @@ SPDX-License-Identifier: GPL-2.0-or-later
             <label class="bm-label">Bandwidth (Hz)
               <input type="number" id="bm-bw" class="status-input" min="0" placeholder="optional" />
             </label>
+            <label class="bm-label" id="bm-dig-sideband-label" style="display:none;">DIG sideband
+              <select id="bm-dig-sideband" class="status-input">
+                <option value="">Global setting</option>
+                <option value="auto">Auto (by band)</option>
+                <option value="usb">USB</option>
+                <option value="lsb">LSB</option>
+              </select>
+            </label>
             <label class="bm-label">Locator
               <input type="text" id="bm-locator" class="status-input" maxlength="6" placeholder="e.g. JO93" />
             </label>