[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-core/src/radio/freq.rs

@@ -7,7 +7,7 @@ use serde::{Deserialize, Serialize};
 const SPEED_OF_LIGHT_M_PER_S: f64 = 299_792_458.0;
 
 /// Supported band range in Hz.
-#[derive(Debug, Clone, Serialize, Deserialize)]
+#[derive(Debug, Clone, Serialize, Deserialize, PartialEq, Eq)]
 pub struct Band {
     pub low_hz: u64,
     pub high_hz: u64,
@@ -23,7 +23,7 @@ impl Band {
 }
 
 /// Frequency wrapper (Hz).
-#[derive(Debug, Clone, Copy, Serialize, Deserialize)]
+#[derive(Debug, Clone, Copy, Serialize, Deserialize, PartialEq, Eq)]
 pub struct Freq {
     pub hz: u64,
 }

src/trx-core/src/rig/mod.rs

@@ -21,14 +21,14 @@ pub mod response;
 pub mod state;
 
 /// How this backend communicates with the rig.
-#[derive(Debug, Clone, Serialize, Deserialize)]
+#[derive(Debug, Clone, Serialize, Deserialize, PartialEq, Eq)]
 pub enum RigAccessMethod {
     Serial { path: String, baud: u32 },
     Tcp { addr: String },
 }
 
 /// Static info describing a rig backend.
-#[derive(Debug, Clone, Serialize, Deserialize)]
+#[derive(Debug, Clone, Serialize, Deserialize, PartialEq)]
 pub struct RigInfo {
     pub manufacturer: String,
     pub model: String,
@@ -37,7 +37,7 @@ pub struct RigInfo {
     pub access: RigAccessMethod,
 }
 
-#[derive(Debug, Clone, Serialize, Deserialize)]
+#[derive(Debug, Clone, Serialize, Deserialize, PartialEq, Eq)]
 pub struct RigCapabilities {
     #[serde(default = "default_min_freq_step_hz")]
     pub min_freq_step_hz: u64,
@@ -233,7 +233,7 @@ pub trait RigCat: Rig + Send {
 }
 
 /// Snapshot of a rig's status that every backend can expose.
-#[derive(Debug, Clone, Serialize, Deserialize)]
+#[derive(Debug, Clone, Serialize, Deserialize, PartialEq)]
 pub struct RigStatus {
     pub freq: Freq,
     pub mode: RigMode,
@@ -249,21 +249,21 @@ pub trait RigStatusProvider {
     fn status(&self) -> RigStatus;
 }
 
-#[derive(Debug, Clone, Serialize, Deserialize)]
+#[derive(Debug, Clone, Serialize, Deserialize, PartialEq)]
 pub struct RigVfo {
     pub entries: Vec<RigVfoEntry>,
     /// Index into `entries` for the active VFO, if known.
     pub active: Option<usize>,
 }
 
-#[derive(Debug, Clone, Serialize, Deserialize)]
+#[derive(Debug, Clone, Serialize, Deserialize, PartialEq)]
 pub struct RigVfoEntry {
     pub name: String,
     pub freq: Freq,
     pub mode: Option<RigMode>,
 }
 
-#[derive(Debug, Clone, Serialize, Deserialize)]
+#[derive(Debug, Clone, Serialize, Deserialize, PartialEq)]
 pub struct RigTxStatus {
     pub power: Option<u8>,
     pub limit: Option<u8>,
@@ -271,13 +271,13 @@ pub struct RigTxStatus {
     pub alc: Option<u8>,
 }
 
-#[derive(Debug, Clone, Serialize, Deserialize)]
+#[derive(Debug, Clone, Serialize, Deserialize, PartialEq, Eq)]
 pub struct RigRxStatus {
     pub sig: Option<i32>,
 }
 
 /// Configurable control settings that can be pushed to the rig.
-#[derive(Debug, Clone, Serialize)]
+#[derive(Debug, Clone, Serialize, PartialEq)]
 pub struct RigControl {
     pub enabled: Option<bool>,
     pub lock: Option<bool>,

src/trx-core/src/rig/state.rs

@@ -8,7 +8,7 @@ use crate::radio::freq::Freq;
 use crate::rig::{RigControl, RigInfo, RigRxStatus, RigStatus, RigStatusProvider, RigTxStatus};
 
 /// Simple transceiver state representation held by the rig task.
-#[derive(Debug, Clone, Serialize)]
+#[derive(Debug, Clone, Serialize, PartialEq)]
 pub struct RigState {
     #[serde(skip_deserializing)]
     pub rig_info: Option<RigInfo>,
@@ -280,7 +280,7 @@ impl RigState {
 }
 
 /// Current filter/DSP state for backends that support runtime filter adjustment.
-#[derive(Debug, Clone, Serialize, Deserialize)]
+#[derive(Debug, Clone, Serialize, Deserialize, PartialEq)]
 pub struct RigFilterState {
     pub bandwidth_hz: u32,
     pub fir_taps: u32,
@@ -324,7 +324,7 @@ fn default_wfm_denoise_level() -> WfmDenoiseLevel {
 }
 
 /// Spectrum data from SDR backends (FFT magnitude over the full capture bandwidth).
-#[derive(Debug, Clone, Serialize, Deserialize)]
+#[derive(Debug, Clone, Serialize, Deserialize, PartialEq)]
 pub struct SpectrumData {
     /// FFT magnitude bins in dBFS, FFT-shifted so DC (centre frequency) is at index N/2.
     pub bins: Vec<f32>,
@@ -338,7 +338,7 @@ pub struct SpectrumData {
 }
 
 /// Live RDS metadata decoded from a WFM broadcast.
-#[derive(Debug, Clone, Default, Serialize, Deserialize)]
+#[derive(Debug, Clone, Default, Serialize, Deserialize, PartialEq)]
 pub struct RdsData {
     #[serde(default, skip_serializing_if = "Option::is_none")]
     pub pi: Option<u16>,
@@ -371,7 +371,7 @@ pub struct RdsData {
 }
 
 /// Read-only projection of state shared with clients.
-#[derive(Debug, Clone, Serialize, Deserialize)]
+#[derive(Debug, Clone, Serialize, Deserialize, PartialEq)]
 pub struct RigSnapshot {
     pub info: RigInfo,
     pub status: RigStatus,