feat(backfill): --reparse-txt flag to refresh bw_report from preserved .TXT

The existing backfill_sidecars.py PRESERVES the bw_report block across
regenerations — it's treated as the source of truth from the original
ingest pass (the .TXT isn't reachable from the script's normal data
path, so it can't be re-derived).

That means parser-side fixes (like the 2026-05-28 ">100 Hz" ZC Freq
addition) won't reach old events even with --force.  The new
--reparse-txt flag fixes that: when the sidecar's source.txt_filename
points at a preserved <serial>/<filename>_ASCII.TXT, the script re-runs
the current parser against it and overwrites the bw_report block.

Implies sidecar regeneration on every event (bypasses the
sha-up-to-date / version-up-to-date skip), so that the .h5 cascade-
regenerates alongside.  No-op for events without a preserved .TXT
(legacy ingests pre-2026-05-27).  Idempotent — re-running it produces
the same sidecar bytes when the parser hasn't changed.

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

scripts/backfill_sidecars.py

@@ -103,6 +103,17 @@ def main(argv=None) -> int:
             "STRT-rectime byte-offset fix in v0.15.x)."
         ),
     )
+    p.add_argument(
+        "--reparse-txt", action="store_true",
+        help=(
+            "Re-parse the preserved <serial>/<filename>_ASCII.TXT with the "
+            "current bw_ascii_report parser and overwrite the sidecar's "
+            "bw_report block.  Use this after upgrading the ASCII parser to "
+            "pull in new fields (e.g. zc_freq_above_range for BW '>100 Hz' "
+            "ZC peaks).  No-op for events without a preserved .TXT; safely "
+            "idempotent when the parser hasn't changed."
+        ),
+    )
     p.add_argument("-v", "--verbose", action="store_true")
     args = p.parse_args(argv)
 
@@ -153,7 +164,7 @@ def main(argv=None) -> int:
             # of the sidecar implies staleness of the derived .h5 (both
             # come out of the same decoder).
             sidecar_stale = True
-            if sidecar_path.exists() and not args.force:
+            if sidecar_path.exists() and not args.force and not args.reparse_txt:
                 try:
                     existing = event_file_io.read_sidecar(sidecar_path)
                     sha_ok = existing.get("blastware", {}).get("sha256") == bw_sha
@@ -314,6 +325,24 @@ def main(argv=None) -> int:
                     except Exception:
                         pass
 
+                # --reparse-txt: if a .TXT is preserved on disk, run the
+                # current parser against it and overwrite the bw_report
+                # block.  Picks up post-ingest parser fixes (e.g. the
+                # 2026-05-28 zc_freq_above_range / ">100 Hz" addition).
+                if args.reparse_txt and preserved_txt_fn:
+                    try:
+                        from minimateplus import bw_ascii_report
+                        txt_path = store.txt_path_for(serial, path.name)
+                        if txt_path.exists():
+                            refreshed = bw_ascii_report.parse_report_file(txt_path)
+                            preserved_bw_report = event_file_io._bw_report_to_dict(refreshed)
+                            log.debug("reparsed bw_report from %s", txt_path.name)
+                        else:
+                            log.debug("--reparse-txt: no .TXT at %s (sidecar says %r)",
+                                      txt_path, preserved_txt_fn)
+                    except Exception as exc:
+                        log.warning("--reparse-txt failed for %s: %s", path.name, exc)
+
                 # Overlay BW ASCII report fields onto the rebuilt Event
                 # BEFORE the sidecar + DB write.  Mirrors what the ingest
                 # path does — BW's reported peaks (and sample_rate /