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/event_browser.html

@@ -717,8 +717,9 @@ function renderWaveform(data) {
     // up AND down).  Mic + histograms keep default auto-scale (always
     // positive values; 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 for full shape detail.
       let absMax = 0;
       for (const v of values) {
         const a = Math.abs(v);
@@ -726,13 +727,25 @@ function renderWaveform(data) {
       }
       const padded = (absMax || 1) * 1.10;
       yBounds = { min: -padded, max: padded };
+    } else if (isGeo && isHistogram) {
+      // Histogram geo: enforce minimum chart range so quiet events
+      // look quiet (matches BW's near-fixed-scale convention).
+      const HIST_GEO_MIN_INS = 0.05;
+      let p = 0;
+      for (const v of values) { const a = Math.abs(v); if (a > p) p = a; }
+      yBounds = { min: 0, max: Math.max(p * 1.10, HIST_GEO_MIN_INS) };
     } else if (ch === 'MicL' && micUnit === 'dBL') {
-      // Baseline at noise-floor minimum (matches what we floored
-      // null/quiet samples to), top at peak + 5 dB headroom.
+      // Mic dBL: baseline at noise-floor minimum, top at peak + 5 dB.
       const peakDbl = (typeof peak === 'number' && isFinite(peak))
         ? peak + 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.
+      const HIST_MIC_MIN_PSI = 0.001;
+      let p = 0;
+      for (const v of values) { const a = Math.abs(v); if (a > p) p = a; }
+      yBounds = { min: 0, max: Math.max(p * 1.10, HIST_MIC_MIN_PSI) };
     }
 
     const chart = new Chart(canvas, {