viewers: enforce minimum Y-range on histogram channels

Quiet histogram events were filling the chart panel even though the
peak was tiny (0.005 in/s rendered as 90% of chart height because
Chart.js auto-scaled to peak * 1.1).  Made everything look uniformly
loud regardless of actual amplitude.

BW's solution: a near-fixed scale per channel ("Geo: 0.002 in/s/div"
from the footer).  Quiet events render small, loud events render
proportionally tall.

Match the intent without copying BW's "no Y-axis labels at all"
convention.  For histogram channels:

  Geo (in/s):       min Y range 0.05 in/s
  Mic in psi:       min Y range 0.001 psi
  Mic in dBL:       unchanged (the 60 dBL floor + peak+5 top already
                    gives quiet events a sensible baseline)

So a 0.005 in/s geo event renders as ~10% of chart height; a 0.05
event fills it; a 5.0 event still fills it (max(peak*1.1, 0.05) ==
peak*1.1 for any peak > 0.045).

Waveform charts unchanged — they should zoom for shape detail.
Applied to both the modal in sfm_webapp.html and the standalone
/events page in event_browser.html.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

sfm/sfm_webapp.html

@@ -2748,8 +2748,9 @@ function _renderScWaveform(data) {
     //   - Mic (always positive sound pressure) + histograms (per-interval
     //     peaks, always positive): default auto-scale, zero at the bottom.
     let yBounds = {};
-    const isGeoWaveform = !isHistogram && ch !== 'MicL';
-    if (isGeoWaveform) {
+    const isGeo = ch !== 'MicL';
+    if (isGeo && !isHistogram) {
+      // Waveform geo: symmetric around zero, full zoom to shape detail.
       let absMax = 0;
       for (const v of values) {
         const a = Math.abs(v);
@@ -2757,13 +2758,31 @@ function _renderScWaveform(data) {
       }
       const padded = (absMax || 1) * 1.10;
       yBounds = { min: -padded, max: padded };
+    } else if (isGeo && isHistogram) {
+      // Histogram geo: enforce a minimum chart range so a quiet
+      // 0.005 in/s event renders as ~10% of chart height instead of
+      // filling the panel.  Matches BW's near-fixed-scale convention
+      // (their footer is "Geo: 0.002 in/s/div" — a chart-relative scale,
+      // not auto-zoom).
+      const HIST_GEO_MIN_INS = 0.05;
+      let peak = 0;
+      for (const v of values) { const a = Math.abs(v); if (a > peak) peak = a; }
+      yBounds = { min: 0, max: Math.max(peak * 1.10, HIST_GEO_MIN_INS) };
     } else if (ch === 'MicL' && micUnit === 'dBL') {
-      // Pin baseline at the chart floor (which matches what we flooded
-      // null/quiet samples to), top at the actual peak + a few dB headroom.
+      // Mic in dBL — pin baseline at noise-floor minimum (where we floored
+      // quiet samples), top at actual peak + a few dB headroom.
       const peakDbl = (typeof chPeak === 'number' && isFinite(chPeak))
         ? chPeak + 5
         : 100;
       yBounds = { min: MIC_DBL_FLOOR, max: Math.max(peakDbl, MIC_DBL_FLOOR + 20) };
+    } else if (ch === 'MicL' && isHistogram && micUnit === 'psi') {
+      // Mic histogram in psi — same minimum-range treatment as geo.
+      // 0.001 psi ≈ 110 dBL — typical "loud" mic peak.  Quiet events
+      // sit near the bottom.
+      const HIST_MIC_MIN_PSI = 0.001;
+      let peak = 0;
+      for (const v of values) { const a = Math.abs(v); if (a > peak) peak = a; }
+      yBounds = { min: 0, max: Math.max(peak * 1.10, HIST_MIC_MIN_PSI) };
     }
 
     _scCharts[ch] = new Chart(canvas, {