viewers: smoother mic dBL chart + restore binary/TXT download links

Two issues spotted in the modal:

1. Mic dBL chart looked spikey/discontinuous — isolated bars at 80-95
   with gaps in between.  Cause: _psiToDbl() returns null for zero or
   negative samples, and most mic samples on a quiet event sit at the
   digitization noise floor where they're effectively zero.  Result:
   the chart only renders the moments when instantaneous SPL exceeded
   the Y-axis bottom — looks like a sound trigger gate.

   Fix: new _psiToDblForChart() rectifies the AC waveform (abs), then
   converts to dBL, then floors at MIC_DBL_FLOOR=60 dBL.  Chart now
   has a continuous 60 dBL baseline with peaks above it — matches how
   acoustic engineers expect SPL-vs-time.  Y-axis bottom pinned to
   MIC_DBL_FLOOR, top to peak + 5 dB headroom.  Peak label still uses
   the unrectified _psiToDbl so the displayed peak value is exact.

2. Filename in Source/Files block was unlinked.  Endpoint exists
   (/db/events/{id}/blastware_file) — just wasn't wired to the modal.
   Made it a clickable download link.  Same treatment for the
   preserved .TXT — added "(download .TXT)" link next to source kind
   when source.txt_filename is populated (events ingested after the
   .TXT preservation feature landed; older events show no link).

Applied to both the inline 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

@@ -356,6 +356,18 @@ function _psiToDbl(psi) {
   return 20 * Math.log10(psi / DBL_REF);
 }
 
+// Per-sample mic chart conversion — rectify the AC waveform, dBL,
+// floor below the noise-floor minimum.  Gives a continuous baseline
+// instead of the spikey/discontinuous look you get from raw _psiToDbl.
+const MIC_DBL_FLOOR = 60;
+function _psiToDblForChart(psi) {
+  if (psi == null) return MIC_DBL_FLOOR;
+  const a = Math.abs(psi);
+  if (a === 0) return MIC_DBL_FLOOR;
+  const dbl = 20 * Math.log10(a / DBL_REF);
+  return dbl > MIC_DBL_FLOOR ? dbl : MIC_DBL_FLOOR;
+}
+
 // Format an ISO timestamp in the browser's local timezone — UTC values
 // (with 'Z' suffix) convert; naive values are interpreted as local clock.
 // Returns '—' for null/empty/unparseable.
@@ -638,7 +650,10 @@ function renderWaveform(data) {
     let peak  = chData.peak;
     const peakT = chData.peak_t_ms;
     if (ch === 'MicL' && unit === 'psi' && micUnit === 'dBL') {
-      values = values.map(_psiToDbl);
+      // Per-sample chart uses rectified-and-floored conversion so the
+      // baseline is continuous; the peak label uses the unrectified
+      // converter to preserve the true measurement.
+      values = values.map(_psiToDblForChart);
       peak   = _psiToDbl(peak);
       unit   = 'dB(L)';
     }
@@ -711,6 +726,13 @@ function renderWaveform(data) {
       }
       const padded = (absMax || 1) * 1.10;
       yBounds = { min: -padded, max: padded };
+    } 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.
+      const peakDbl = (typeof peak === 'number' && isFinite(peak))
+        ? peak + 5
+        : 100;
+      yBounds = { min: MIC_DBL_FLOOR, max: Math.max(peakDbl, MIC_DBL_FLOOR + 20) };
     }
 
     const chart = new Chart(canvas, {