ingest: preserve raw BW ASCII report (.TXT) alongside the binary

Previously the .TXT was parsed into the sidecar's bw_report projection
and then discarded at ingest time.  Now save_imported_bw() writes it
to <store>/<serial>/<filename>_ASCII.TXT permanently.

Rationale: with BW Mail / Forwarding Agent being phased out of the
operator workflow, the XML/PDF/WMF those tools produce won't be
available — the binary + .TXT (created by BW ACH itself) are our
only authoritative inputs going forward.  Keeping the raw .TXT
unlocks:

  - Parser bug fixes can be applied RETROACTIVELY by re-parsing the
    stored .TXT, instead of requiring a re-forward from the watcher
    PC (which lost the .TXT after BW ACH cleanup).
  - Audit trail of what BW actually sent us, for debugging.
  - The five known parser-PPV-miss events will be re-parseable once
    the regex fix lands (instead of staying broken indefinitely).

Storage cost: ~15 KB per event × 14k events = ~210 MB on the
existing prod corpus.  Negligible.

Implementation:
  - WaveformStore gains txt_path_for() + open_txt()
  - save_imported_bw() writes the .TXT when bw_report_text is supplied
  - sidecar source block records the txt_filename
  - backfill_sidecars.py preserves txt_filename across regens
  - New GET /db/events/{id}/ascii_report.txt endpoint serves it
  - Returns 404 for events ingested before this change (no .TXT in
    the store yet) — re-forward to populate

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

sfm/waveform_store.py

@@ -108,11 +108,30 @@ class WaveformStore:
         """Return absolute path to the .h5 clean-waveform file for a given event."""
         return self._serial_dir(serial) / f"{filename}.h5"
 
+    def txt_path_for(self, serial: str, filename: str) -> Path:
+        """Return absolute path to the preserved BW ASCII report (.TXT)
+        for a given event.
+
+        We name it ``<filename>_ASCII.TXT`` to match BW's own filename
+        convention in the ACH folder.  Saved at ingest time alongside
+        the binary so the parser bug fixes can be applied retroactively
+        by re-parsing without needing to re-forward from the watcher PC.
+        """
+        return self._serial_dir(serial) / f"{filename}_ASCII.TXT"
+
     def open_blastware(self, serial: str, filename: str) -> Optional[Path]:
         """Return absolute path to an existing event file or None."""
         bw_path, _ = self.paths_for(serial, filename)
         return bw_path if bw_path.exists() else None
 
+    def open_txt(self, serial: str, filename: str) -> Optional[Path]:
+        """Return absolute path to the preserved BW ASCII report for an
+        event, or None if the .TXT wasn't saved at ingest time (events
+        ingested before .TXT preservation landed will show None until
+        re-forwarded)."""
+        p = self.txt_path_for(serial, filename)
+        return p if p.exists() else None
+
     # ── save / load ─────────────────────────────────────────────────────────────
 
     def save(
@@ -357,6 +376,28 @@ class WaveformStore:
         filesize = bw_path.stat().st_size
         sha256   = event_file_io.file_sha256(bw_path)
 
+        # 1b. preserve the raw BW ASCII report (.TXT) alongside the binary.
+        # Saved at <root>/<serial>/<filename>_ASCII.TXT.  Lets us re-parse
+        # offline after parser fixes without needing to re-forward from
+        # the watcher PC.  Negligible storage cost (~15 KB per event).
+        # Skipped silently when no report was supplied (live download path,
+        # manual upload without paired TXT).
+        txt_filename: Optional[str] = None
+        if bw_report_text is not None:
+            try:
+                txt_path = self.txt_path_for(serial, filename)
+                if isinstance(bw_report_text, bytes):
+                    txt_path.write_bytes(bw_report_text)
+                else:
+                    txt_path.write_text(bw_report_text)
+                txt_filename = txt_path.name
+            except Exception as exc:
+                log.warning(
+                    "save_imported_bw: failed to save TXT for %s: %s — "
+                    "continuing without it",
+                    filename, exc,
+                )
+
         # 2. write the .h5 clean-waveform file from the parsed Event.
         # Note: peaks here are computed from raw samples (the BW file
         # doesn't carry the device-authoritative 0C peaks).  Best-effort.
@@ -393,6 +434,7 @@ class WaveformStore:
             blastware_sha256=sha256,
             source_kind="bw-import",
             a5_pickle_filename=None,
+            txt_filename=txt_filename,
             review=existing_review,
             bw_report=bw_report,
         )