[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"] }

src/trx-client/trx-frontend/src/lib.rs

@@ -44,16 +44,25 @@ pub struct SharedSpectrum {
     // Arc so that each SSE client gets a cheap pointer clone instead of
     // copying the entire bin vector (~8 KB for 2048 f32 bins).
     frame: Option<Arc<SpectrumData>>,
+    // RDS JSON serialised once at ingestion; avoids per-client serde work
+    // on every 40 ms tick for a field that changes at most once per second.
+    rds_json: Option<String>,
 }
 
 impl SharedSpectrum {
     pub fn replace(&mut self, frame: Option<SpectrumData>) {
         self.revision = self.revision.wrapping_add(1);
+        self.rds_json = frame
+            .as_ref()
+            .and_then(|f| f.rds.as_ref())
+            .and_then(|r| serde_json::to_string(r).ok());
         self.frame = frame.map(Arc::new);
     }
 
-    pub fn snapshot(&self) -> (u64, Option<Arc<SpectrumData>>) {
-        (self.revision, self.frame.clone())
+    /// Returns `(revision, frame, rds_json)`.
+    /// `rds_json` is pre-serialised; `None` means no RDS data.
+    pub fn snapshot(&self) -> (u64, Option<Arc<SpectrumData>>, Option<String>) {
+        (self.revision, self.frame.clone(), self.rds_json.clone())
     }
 }
 

src/trx-client/trx-frontend/trx-frontend-http/src/api.rs

@@ -424,16 +424,13 @@ pub async fn spectrum(
                 let next = context.spectrum.lock().ok().map(|g| g.snapshot());
 
                 let sse_chunk: Option<String> = match next {
-                    Some((revision, _frame)) if last_revision == Some(revision) => None,
-                    Some((revision, Some(frame))) => {
+                    Some((revision, _frame, _rds)) if last_revision == Some(revision) => None,
+                    Some((revision, Some(frame), rds_json)) => {
                         last_revision = Some(revision);
                         let mut chunk =
                             format!("event: b\ndata: {}\n\n", encode_spectrum_frame(&frame));
-                        // Append an `rds` event only when the RDS payload changes.
-                        let rds_json = frame
-                            .rds
-                            .as_ref()
-                            .and_then(|r| serde_json::to_string(r).ok());
+                        // rds_json is pre-serialised at ingestion; append an
+                        // `rds` event only when the payload changed for this client.
                         if rds_json != last_rds_json {
                             let data = rds_json.as_deref().unwrap_or("null");
                             chunk.push_str(&format!("event: rds\ndata: {data}\n\n"));
@@ -441,7 +438,7 @@ pub async fn spectrum(
                         }
                         Some(chunk)
                     }
-                    Some((revision, None)) => {
+                    Some((revision, None, _)) => {
                         last_revision = Some(revision);
                         Some("data: null\n\n".to_string())
                     }