[feat](trx-frontend-http): /meter SSE endpoint for instant signal metering

Adds a dedicated /meter SSE stream that wraps the per-rig meter watch
and emits one compact JSON frame per update with no equality gating, so
30 Hz samples reach the browser unthrottled. Registered as a Read-access
route. app.js opens a dedicated EventSource on /meter alongside /events,
writing directly to the signal bar and value on each frame with no
requestAnimationFrame coalescing, starts/stops with connect/disconnect,
and reconnects on rig switch.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
Signed-off-by: Stan Grams <sjg@haxx.space>

src/trx-client/trx-frontend/trx-frontend-http/assets/web/app.js

@@ -3700,6 +3700,8 @@ function connect() {
   if (esHeartbeat) {
     clearInterval(esHeartbeat);
   }
+  stopMeterStreaming();
+  startMeterStreaming();
   pollFreshSnapshot();
   const eventsUrl = lastActiveRigId
     ? `/events?remote=${encodeURIComponent(lastActiveRigId)}`
@@ -3778,6 +3780,7 @@ function disconnect() {
     decodeSource = null;
   }
   stopSpectrumStreaming();
+  stopMeterStreaming();
   // Clear timers
   if (esHeartbeat) {
     clearInterval(esHeartbeat);
@@ -3900,6 +3903,9 @@ async function switchRigFromSelect(selectEl) {
   // Reconnect spectrum SSE to the new rig's spectrum channel.
   stopSpectrumStreaming();
   startSpectrumStreaming();
+  // Reconnect meter SSE to the new rig's meter channel.
+  stopMeterStreaming();
+  startMeterStreaming();
   // Reconnect audio to the new rig if audio is active.
   if (rxActive) {
     stopRxAudio();
@@ -6476,6 +6482,8 @@ const spectrumCenterLeftBtn = document.getElementById("spectrum-center-left-btn"
 const spectrumCenterRightBtn = document.getElementById("spectrum-center-right-btn");
 let spectrumSource = null;
 let spectrumReconnectTimer = null;
+let meterSource = null;
+let meterReconnectTimer = null;
 let spectrumDrawPending = false;
 let spectrumAxisKey = "";
 let spectrumDbAxisKey = "";
@@ -6996,6 +7004,63 @@ function stopSpectrumStreaming() {
   clearSpectrumCanvas();
 }
 
+// ── /meter (fast signal-strength) streaming ─────────────────────────────────
+// Dedicated SSE channel pushed at ~30 Hz by trx-server; bypasses /events so
+// meter frames are never gated by full-RigState diffing. Synchronous DOM
+// write per frame — no rAF coalescing, per user requirement that it "feel
+// instant" on the frontend.
+function scheduleMeterReconnect() {
+  if (meterReconnectTimer !== null) return;
+  meterReconnectTimer = setTimeout(() => {
+    meterReconnectTimer = null;
+    startMeterStreaming();
+  }, 1000);
+}
+
+function applyMeterSample(dbm) {
+  if (typeof dbm !== "number" || !Number.isFinite(dbm)) return;
+  prevRenderData.sigDbm = dbm;
+  const sUnits = dbmToSUnits(dbm);
+  sigLastSUnits = sUnits;
+  sigLastDbm = dbm;
+  const pct = sUnits <= 9 ? Math.max(0, Math.min(100, (sUnits / 9) * 100)) : 100;
+  if (signalBar) signalBar.style.width = `${pct}%`;
+  if (signalValue) signalValue.innerHTML = formatSignal(sUnits);
+  refreshSigStrengthDisplay();
+}
+
+function startMeterStreaming() {
+  if (meterSource !== null) return;
+  const url = lastActiveRigId
+    ? `/meter?remote=${encodeURIComponent(lastActiveRigId)}`
+    : "/meter";
+  meterSource = new EventSource(url);
+  meterSource.onmessage = (evt) => {
+    try {
+      const { sig } = JSON.parse(evt.data);
+      applyMeterSample(sig);
+    } catch (_) {}
+  };
+  meterSource.onerror = () => {
+    if (meterSource) {
+      meterSource.close();
+      meterSource = null;
+    }
+    scheduleMeterReconnect();
+  };
+}
+
+function stopMeterStreaming() {
+  if (meterSource !== null) {
+    meterSource.close();
+    meterSource = null;
+  }
+  if (meterReconnectTimer !== null) {
+    clearTimeout(meterReconnectTimer);
+    meterReconnectTimer = null;
+  }
+}
+
 // ── Rendering ────────────────────────────────────────────────────────────────
 function clearSpectrumCanvas() {
   if (!spectrumCanvas || !spectrumGl || !spectrumGl.ready) return;

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

@@ -574,6 +574,7 @@ pub fn configure(cfg: &mut web::ServiceConfig) {
         // SSE streams
         .service(sse::events)
         .service(sse::spectrum)
+        .service(sse::meter)
         // Decoder endpoints
         .service(decoder::decoder_registry)
         .service(decoder::decode_history)

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

@@ -19,6 +19,7 @@ use uuid::Uuid;
 
 use trx_core::RigState;
 use trx_frontend::FrontendRuntimeContext;
+use trx_protocol::MeterUpdate;
 
 use crate::server::vchan::ClientChannelManager;
 
@@ -337,6 +338,62 @@ pub async fn events(
         .streaming(stream))
 }
 
+// ============================================================================
+// /meter SSE endpoint (fast signal-strength stream, ~30 Hz)
+// ============================================================================
+
+fn encode_meter_frame(update: &MeterUpdate) -> String {
+    // Compact JSON: one-line SSE frame, flushed immediately.
+    // Shape: {"sig":-72.3,"ts":12345}
+    format!(
+        "data: {{\"sig\":{:.2},\"ts\":{}}}\n\n",
+        update.sig_dbm, update.ts_ms
+    )
+}
+
+/// SSE stream for per-rig signal-strength updates.
+///
+/// Pushed from the server's per-rig meter broadcast; intentionally bypasses
+/// the `/events` RigState path so high-rate meter samples are never gated by
+/// full-state diffing. Each watch update produces exactly one SSE frame.
+#[get("/meter")]
+pub async fn meter(
+    query: web::Query<RemoteQuery>,
+    context: web::Data<Arc<FrontendRuntimeContext>>,
+) -> Result<HttpResponse, Error> {
+    let rig_id = query.remote.clone().filter(|s| !s.is_empty()).or_else(|| {
+        context
+            .routing
+            .active_rig_id
+            .lock()
+            .ok()
+            .and_then(|g| g.clone())
+    });
+
+    let rx = match rig_id.as_deref() {
+        Some(rid) => context.rig_meter_rx(rid),
+        None => return Ok(HttpResponse::NotFound().finish()),
+    };
+
+    let updates = WatchStream::new(rx).filter_map(|maybe| {
+        let chunk = maybe.as_ref().map(encode_meter_frame);
+        std::future::ready(chunk.map(|s| Ok::<Bytes, Error>(Bytes::from(s))))
+    });
+
+    // Infrequent keepalive comment; real meter frames carry the heartbeat.
+    let pings = IntervalStream::new(time::interval(Duration::from_secs(15)))
+        .map(|_| Ok::<Bytes, Error>(Bytes::from(": ping\n\n")));
+
+    let stream = select(pings, updates);
+
+    Ok(HttpResponse::Ok()
+        .insert_header((header::CONTENT_TYPE, "text/event-stream"))
+        .insert_header((header::CONTENT_ENCODING, "identity"))
+        .insert_header((header::CACHE_CONTROL, "no-cache"))
+        .insert_header((header::CONNECTION, "keep-alive"))
+        .streaming(stream))
+}
+
 // ============================================================================
 // /spectrum SSE endpoint
 // ============================================================================

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

@@ -526,6 +526,7 @@ impl RouteAccess {
             || path == "/decode"
             || path == "/decode/history"
             || path == "/spectrum"
+            || path == "/meter"
             || path == "/audio"
             || path == "/bookmarks"
             || path.starts_with("/status?")
@@ -534,6 +535,7 @@ impl RouteAccess {
             || path.starts_with("/decode?")
             || path.starts_with("/decode/history?")
             || path.starts_with("/spectrum?")
+            || path.starts_with("/meter?")
             || path.starts_with("/audio?")
             || path.starts_with("/bookmarks?")
             || path.starts_with("/bookmarks/")
@@ -703,6 +705,7 @@ mod tests {
         assert_eq!(RouteAccess::from_path("/events"), RouteAccess::Read);
         assert_eq!(RouteAccess::from_path("/decode"), RouteAccess::Read);
         assert_eq!(RouteAccess::from_path("/spectrum"), RouteAccess::Read);
+        assert_eq!(RouteAccess::from_path("/meter"), RouteAccess::Read);
         assert_eq!(RouteAccess::from_path("/audio"), RouteAccess::Read);
     }