[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/frontend/tests/spectrum-layout.mjs

@@ -35,11 +35,16 @@ const BANDPLAN = {
 const BAND_WITH_CONTENT = 7074000;
 const BAND_WITHOUT_CONTENT = 14074000;
 
+// The meter sits well away from the spectrum's noise floor, so a squelch that
+// took its level from the plot would land somewhere else entirely.
+const METER_DB = -70;
+
 const fixture = await startWebFixture({
   spectrum: true,
   bookmarks: BOOKMARKS,
   bandplan: BANDPLAN,
   bandplanEnabled: true,
+  meterDb: METER_DB,
 });
 const { browser, page, runtimeErrors } = await startBrowser(chromium);
 
@@ -125,10 +130,14 @@ try {
   // row, with nothing on screen to relate the number to.
   await page.locator("summary", { hasText: "Audio controls" }).click();
   await page.locator("#sdr-squelch-toggle").click();
-  // Auto parks it just above the noise, which is mid-axis and leaves room to
-  // drag in either direction.
+  // Auto parks it just above the noise the meter reports — not above the
+  // spectrum's noise floor, which sits anywhere from 1 dB below to 22 dB above
+  // the meter depending on span, bandwidth and decimation.
   await page.locator("#sdr-squelch-auto").click();
-  await page.waitForTimeout(300);
+  await page.waitForTimeout(400);
+  const auto = await page.evaluate(() => Number(document.getElementById("sdr-squelch-db").value));
+  assert.ok(Math.abs(auto - (METER_DB + 5)) <= 1,
+    `auto put the threshold at ${auto} dB with the meter at ${METER_DB} dB`);
   const squelchOn = await page.evaluate(() => {
     const line = document.getElementById("spectrum-squelch-line");
     return {