[fix](trx-frontend-http): measure auto squelch from the meter

Auto took the spectrum's noise floor and added 6 dB, but the threshold
is compared against the channel level the meter reports, and the two sit
a long way apart: the gap is set by the FFT size and window, the channel
bandwidth, the decimation, and peak-versus-mean statistics.  Measured on
white noise it runs +22.1 dB at 48k/8k/3k, +18.7 dB at 240k/24k/12k and
-1.2 dB at 1.92M/24k/12k — a 23 dB swing across ordinary configurations.
Only the last of those is anywhere near right, so on a narrow span Auto
set the gate some 20 dB below the noise and it never closed.

It now reads the same number the DSP compares: the 20th percentile of
the meter over the last ten seconds, plus 5 dB.  The percentile keeps a
burst of traffic inside the window from dragging the estimate up, and
5 dB clears the meter's own jitter, which measured 0.9-1.6 dB.  Nothing
in it converts between scales, so no part of the signal chain can put it
out again.  With no history yet — a fresh connection, a rig switch — it
listens for a moment rather than refusing.

The fixture gained a streaming /meter, without which there is nothing to
measure, and the spectrum test pins auto to the meter it serves.

Signed-off-by: Stan Grams <sjg@haxx.space>

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

@@ -4727,6 +4727,7 @@ function render(update) {
       const sUnits = dbmToSUnits(update.status.rx.sig);
       sigLastSUnits = sUnits;
       sigLastDbm = update.status.rx.sig;
+      recordSquelchMeterSample(update.status.rx.sig);
       const pct = sUnits <= 9 ? Math.max(0, Math.min(100, sUnits / 9 * 100)) : 100;
       signalBar.style.width = `${pct}%`;
       signalValue.innerHTML = formatSignal(sUnits);
@@ -6369,12 +6370,28 @@ function setSdrSquelch(thresholdDb, enabled, options = {}) {
   renderSdrSquelch();
   if (options.submit !== false) submitSdrSquelch();
 }
+var SQUELCH_NOISE_WINDOW_MS = 1e4;
+var SQUELCH_NOISE_MARGIN_DB = 5;
+var SQUELCH_MEASURE_MS = 1500;
+var squelchMeterSamples = [];
+function recordSquelchMeterSample(db) {
+  if (!isFiniteNumber(db)) return;
+  const now = Date.now();
+  squelchMeterSamples.push({ t: now, v: db });
+  while (squelchMeterSamples[0] && now - squelchMeterSamples[0].t > SQUELCH_NOISE_WINDOW_MS) {
+    squelchMeterSamples.shift();
+  }
+}
+function squelchNoiseFloorDb() {
+  const now = Date.now();
+  const values = squelchMeterSamples.filter((sample) => now - sample.t <= SQUELCH_NOISE_WINDOW_MS).map((sample) => sample.v).sort((a, b) => a - b);
+  if (values.length < 4) return null;
+  return values[Math.floor((values.length - 1) * 0.2)] ?? null;
+}
 function autoSquelchThresholdDb() {
-  const data = lastSpectrumData || window.lastSpectrumData;
-  if (!data || !isNumericBins(data.bins) || data.bins.length === 0) return null;
-  const noiseDb = estimateNoiseFloorDb(data.bins);
-  if (noiseDb == null || !isFiniteNumber(noiseDb)) return null;
-  return clampSdrSquelchDb(noiseDb + 6);
+  const noiseDb = squelchNoiseFloorDb();
+  if (noiseDb == null) return null;
+  return clampSdrSquelchDb(noiseDb + SQUELCH_NOISE_MARGIN_DB);
 }
 function updateSdrSquelchControlVisibility() {
   if (!sdrSquelchWrapEl) return;
@@ -6407,17 +6424,30 @@ if (sdrSquelchToggleBtn) {
     setSdrSquelch(sdrSquelchThresholdDb, !sdrSquelchEnabled);
   });
 }
+function applyAutoSquelch(threshold) {
+  setSdrSquelch(threshold, true);
+  showHint(`Squelch ${threshold} dB`, 1500);
+}
 var sdrSquelchAutoBtn = document.getElementById("sdr-squelch-auto");
 if (sdrSquelchAutoBtn) {
   sdrSquelchAutoBtn.addEventListener("click", () => {
     if (!sdrSquelchSupported) return;
     const threshold = autoSquelchThresholdDb();
-    if (threshold == null) {
-      showHint("No spectrum to measure the noise from", 1800);
+    if (threshold != null) {
+      applyAutoSquelch(threshold);
       return;
     }
-    setSdrSquelch(threshold, true);
-    showHint(`Squelch ${threshold} dB`, 1500);
+    sdrSquelchAutoBtn.disabled = true;
+    showHint("Measuring the noise…");
+    setTimeout(() => {
+      sdrSquelchAutoBtn.disabled = false;
+      const measured = autoSquelchThresholdDb();
+      if (measured == null) {
+        showHint("No meter to measure the noise from", 1800);
+        return;
+      }
+      applyAutoSquelch(measured);
+    }, SQUELCH_MEASURE_MS);
   });
 }
 if (squelchGripEl) {
@@ -8067,6 +8097,7 @@ function flushMeterDom() {
   const sUnits = dbmToSUnits(dbm);
   sigLastSUnits = sUnits;
   sigLastDbm = dbm;
+  recordSquelchMeterSample(dbm);
   const pct = sUnits <= 9 ? Math.max(0, Math.min(100, sUnits / 9 * 100)) : 100;
   if (signalBar) signalBar.style.width = `${pct}%`;
   if (signalValue) signalValue.innerHTML = formatSignal(sUnits);