scripts/backfill_sidecars: skip .h5 write when decoder returned no samples

Discovered while dry-running the backfill on the prod store: ~10,000
of ~10,059 events are histogram-mode (filename extension `*H`), and
the waveform-body codec wired in via the previous commit doesn't
handle histogram-mode bodies — only the waveform-mode codec at
§7.6.1 is implemented; the histogram-mode codec at §7.6.2 of the
protocol reference is documented but no Python implementation
exists yet.

Without this guard, every histogram event's .h5 file would be
*replaced* with an empty one — strictly worse than today's
broken-int16-LE .h5 because any downstream viewer expecting
non-empty sample arrays would now error out instead of just
rendering wrong values.

Fix: after the decoder runs, check whether any channel has samples.
If not, skip the .h5 write entirely.  The sidecar still regenerates
(refreshing the tool_version stamp and any peaks/project info from
the DB row), but the existing .h5 is left untouched.

This is a *temporary* gate.  When the histogram codec lands (next
branch: `feat/wire-histogram-codec`), the has_samples check can be
removed and the backfill will then correctly regenerate all .h5
files, histogram and waveform alike.

Observed effect (dry-run on prod store, 10,059 events):
  - waveform events (~5%): "[DRY ] would write … + .h5 (would (re)write)"
  - histogram events (~95%): "[DRY ] would write … + .h5 (skipped-empty-samples)"
  - sidecar tool_version bump succeeds for both

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

scripts/backfill_sidecars.py

@@ -311,12 +311,32 @@ def main(argv=None) -> int:
                 #     int16-LE codec era — bumping TOOL_VERSION to 0.20.0+
                 #     marks every pre-codec sidecar stale, which now
                 #     correctly cascades to .h5 regeneration too.
+                #
+                # Skip the .h5 write when the decoder couldn't produce
+                # samples — this is the histogram-mode case today
+                # (waveform_codec.decode_waveform_v2 only handles the
+                # waveform-mode body format per §7.6.1; the histogram
+                # codec at §7.6.2 is documented but not yet implemented).
+                # Without this check we'd replace the existing (broken
+                # int16-LE) histogram .h5 with an empty one, which is
+                # arguably worse for any consumer expecting non-empty
+                # sample arrays.  When the histogram codec lands, this
+                # check can come out.
+                has_samples = bool(
+                    ev.raw_samples and any(
+                        ev.raw_samples.get(ch) for ch in ("Tran", "Vert", "Long", "MicL")
+                    )
+                )
                 hdf5_path = store.hdf5_path_for(serial, path.name)
                 hdf5_filename = hdf5_path.name if hdf5_path.exists() else None
                 hdf5_action = "kept"
-                need_h5 = not args.skip_hdf5 and (
+                need_h5 = (
-                    args.force or not hdf5_path.exists() or sidecar_stale
+                    not args.skip_hdf5
+                    and (args.force or not hdf5_path.exists() or sidecar_stale)
+                    and has_samples
                 )
+                if not has_samples and not args.skip_hdf5:
+                    hdf5_action = "skipped-empty-samples"
                 if need_h5:
                     if args.dry_run:
                         hdf5_action = "would (re)write"