histogram aggregation + parser extension for BW interval fields

Three layered changes that together make histogram charts visually
match BW's printout (one bar per interval, not per codec block):

1. bw_ascii_report parser captures histogram fields it previously
   dropped:
     - Histogram Start/Stop Time + Date → datetime
     - Number of Intervals + Interval Size (string + parsed seconds)
     - <Channel> Peak Time + Peak Date → datetime (per-channel)
     - Peak Vector Sum Date (combined with PVS Time → datetime;
       clears the bogus seconds parse that interpreted "22:33:52"
       as 22.0)
   New _parse_iso_date() handles BW's ISO format for histograms
   (waveforms use "May 8, 2026" long form).  New _parse_interval_size()
   handles "1 minute" / "5 minutes" / "15 seconds" etc.

2. _bw_report_to_dict() projects the new fields into a new
   bw_report.histogram block in the sidecar.

3. /db/events/{id}/waveform.json wraps the existing path 1 (HDF5)
   output with _maybe_aggregate_histogram(): when the event is a
   histogram AND the sidecar has bw_report.histogram.n_intervals,
   group the codec's per-block samples into N intervals via
   max-per-group and return the aggregated array.  time_axis gains
   histogram_aggregated / n_intervals / interval_size_s / interval_times
   fields.

Frontend (both modal chart in sfm_webapp.html + standalone event
browser) uses interval_times as x-axis labels when provided (BW-style
HH:MM:SS), falls back to interval index.

Defensive: aggregation is no-op when the sidecar lacks the histogram
block (events ingested before this change).  Activates automatically
on prod once a watcher re-forward populates new sidecars.

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

sfm/sfm_webapp.html

@@ -2684,10 +2684,18 @@ function _renderScWaveform(data) {
     chartsDiv.appendChild(wrap);
 
     // Waveform: per-sample time in ms relative to trigger (negative for pretrig).
-    // Histogram: interval index (1..N); time math doesn't apply to per-interval peaks.
-    const times = isHistogram
-      ? values.map((_, i) => i + 1)
-      : values.map((_, i) => t0Ms + i * dtMs);
+    // Histogram: when the server has aggregated to BW-reported intervals AND
+    // provides per-interval timestamps, use those as x-axis labels (HH:MM:SS).
+    // Falls back to interval index.
+    let times;
+    if (isHistogram) {
+      const intervalTimes = ta.interval_times || [];
+      times = (intervalTimes.length === values.length)
+        ? intervalTimes
+        : values.map((_, i) => i + 1);
+    } else {
+      times = values.map((_, i) => t0Ms + i * dtMs);
+    }
 
     // Downsample for rendering when very long.
     const MAX = 3000;