docs(changelog): record that the 32000 scale fix hit histograms and series-4 too

The scale lives in _samples_to_float, which every event passes through
regardless of source codec, so waveforms, histograms and Thor IDF events
were all 2.34% low — not just waveforms.  Verified after regeneration:
series-3 histogram peaks vs ASCII reports now median 1.0000 across 1137
comparisons (0.9766 under 32768); series-4 peaks vs device peaks moved
from median 0.960 to 0.983 across 1468.

The four block-framing fixes remain waveform-only; histogram_codec is
untouched.

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

CHANGELOG.md

@@ -25,6 +25,16 @@ All notable changes to seismo-relay are documented here.
   The mic path is unaffected — it back-solves its own per-count factor from the
   device-reported peak.
 
+  **Scope:** the scale lives in `_samples_to_float`, which every event passes
+  through regardless of which codec produced the samples — so this affected
+  **waveforms, histograms and series-4 (Thor IDF) alike**, not just waveforms.
+  Verified after regeneration: series-3 histogram peaks vs their ASCII reports
+  now sit at a median ratio of 1.0000 across 1,137 comparisons (0.9766 under
+  32768); series-4 peaks vs device peaks moved from a median 0.960 to 0.983
+  across 1,468 comparisons.  The four block-framing fixes below are
+  waveform-only — histograms decode via `histogram_codec.decode_histogram_body`,
+  which is untouched.
+
 - **Series-3 waveform codec: four block-framing cases caused silent channel
   truncation.** `walk_body` hit its unknown-tag `break` mid-stream and every
   channel decoded after that point came out short — typically Vert/Long/MicL,