fix(backfill): remove stale .h5 when nothing decodes; log the 415-file histogram variant

backfill_sidecars.py skipped the .h5 write when a file produced no
samples, with the stated intent of not replacing it with an empty
placeholder.  That silently preserved output from a superseded decoder.

After the record-chain fix, 415 histogram files stopped decoding (216 on
BE18193, 199 on BE9440) but kept .h5 files whose peaks ran up to 400x
the device-reported PPV.  Those were feeding charts and the
false-trigger detector with nothing marking them.  The .h5 is now
removed in that case and the run reports stale_h5_removed.

Store-wide effect, series-3, decoded peak vs device-reported PPV:
  waveform   1307/1307 (100%), mean abs ratio error 0.00000
  histogram  4434/4435 (100%)
Both were 99% with a tail of 18 and 25 wrong files respectively.

The 415 files are a genuine unmapped format variant, not a regression:
their bodies open `00 00 00 01 0a 00` (valid block header, marker 0a at
[4], block_ctr 256) but block[28:32] matches neither known tail, and no
stride from 8 to 64 bytes places a marker at [4] consistently.  Bodies
are very large (one is 360,573 bytes).  They were previously being
decoded by the WAVEFORM codec, which accepted them and returned garbage
- so the gap pre-dates today's work; the fix only exposed it.  Logged as
an open question in the protocol reference.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HgTe8CamXAHcAmaQ6QNcog

CHANGELOG.md

@@ -8,6 +8,15 @@ All notable changes to seismo-relay are documented here.
 
 ### Fixed
 
+- **`backfill_sidecars.py` now removes a stale `.h5` when nothing decodes.**
+  It previously skipped the write "so we don't replace whatever's there with an
+  empty placeholder", which silently preserved output from a superseded
+  decoder. After the record-chain fix, 415 histogram files stopped decoding (an
+  unmapped block variant on BE18193 and BE9440) but kept `.h5` files whose peaks
+  ran up to **400× the device's own reported PPV** — garbage feeding the charts
+  and the false-trigger detector with nothing marking it. Reports a
+  `stale_h5_removed` count.
+
 - **The series-3 waveform body is a RECORD CHAIN, not a tag stream — this
   supersedes the segment-header model, including the fixes made earlier the
   same day.**