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>

minimateplus/event_file_io.py

@@ -171,6 +171,10 @@ def _bw_report_to_dict(report: BwAsciiReport) -> dict:
             "vector_sum": {
                 "ips":     report.peak_vector_sum_ips,
                 "time_s":  report.peak_vector_sum_time_s,
+                # Histogram events have an absolute date+time for the PVS
+                # (the interval at which it occurred); waveform events
+                # only have the time_s offset.
+                "when":    report.peak_vector_sum_when.isoformat() if report.peak_vector_sum_when else None,
             },
         },
         "mic": {
@@ -185,6 +189,17 @@ def _bw_report_to_dict(report: BwAsciiReport) -> dict:
             "long": _sc("Long"),
             "mic":  _sc("MicL"),
         },
+        # Histogram-specific fields (None on waveform-mode events).
+        # Per-channel absolute peak time/date for histograms — for
+        # waveforms see channels[ch]["time_of_peak_s"] instead.
+        "histogram": {
+            "start":               report.histogram_start.isoformat() if report.histogram_start else None,
+            "stop":                report.histogram_stop.isoformat()  if report.histogram_stop  else None,
+            "n_intervals":         report.histogram_n_intervals,
+            "interval_size":       report.histogram_interval_size_str,
+            "interval_size_s":     report.histogram_interval_size_s,
+            "channel_peak_when":   {ch: dt.isoformat() for ch, dt in report.channel_peak_when.items()},
+        },
         "monitor_log":   monitor_log,
         "pc_sw_version": report.pc_sw_version,
     }