[perf](trx-client): three quick-win optimizations

State deduplication via PartialEq + send_if_modified:
  Derive PartialEq on the full RigState / RigSnapshot type tree
  (Freq, Band, RigInfo, RigCapabilities, RigStatus, RigTxStatus,
  RigRxStatus, RigControl, RigVfo, RigVfoEntry, RigFilterState,
  RdsData, SpectrumData, RigState, RigSnapshot).  Use
  state_tx.send_if_modified() in refresh_remote_snapshot() so
  WatchStream only wakes SSE /events subscribers when state
  actually changed; with a stable rig this eliminates ~1.3
  spurious JSON serialisations per second per connected client.

Cache-remote-rigs skip on unchanged list:
  cache_remote_rigs() was rebuilding the Vec and cloning every
  field on every 750 ms poll.  Add a structural check (rig_id,
  display_name, initialized, audio_port) and return early when
  nothing has changed — the common steady-state case.

RDS JSON pre-serialised at ingestion:
  SharedSpectrum.replace() now serialises the optional RDS object
  once and stores it alongside the Arc<SpectrumData> frame.
  Each /spectrum SSE client's 40 ms tick reads the cached string
  instead of calling serde_json::to_string() per-client per-tick.
  Add serde_json to trx-frontend Cargo.toml to support this.

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

src/trx-client/trx-frontend/Cargo.toml

@@ -9,5 +9,6 @@ edition = "2021"
 
 [dependencies]
 bytes = "1"
+serde_json = { workspace = true }
 trx-core = { path = "../../trx-core" }
 tokio = { workspace = true, features = ["sync"] }