docs: adopt a changelog convention, and make Unreleased follow it

Brian asked what the standard is; there wasn't a written one, only a de facto
pattern in the history. This writes it down in CLAUDE.md and fixes the one
place the repo already diverged from it.

The rule: write the entry in the same commit as the work, under ## Unreleased;
cut the version on dev in a dedicated chore(release) commit; never touch the
changelog at a merge boundary. The entry goes in with the change because that
is the only moment you still know why.

Two additions beyond what the history already did:

No preamble under ## Unreleased. The themed opening paragraph gets written at
release time, when the whole release is visible and can be named honestly. The
current one proved the point — "Blastware Event/FFT-Report parity: the FFT,
the USBM compliance chart, and the sensor self-check" was accurate when the
first item landed and stopped being accurate once rescue-on-connect landed
under the same heading. Removed here; the release commit writes a new one
covering everything actually in the release.

And the operational consequence is now mandatory on any entry touching the
codec, the waveform store, or the DB — including when it is "none". This
repo's changelog is how future-you learns whether a deploy costs two hours on
the NAS, so silence is ambiguous and "none" is information. The old preamble's
load-bearing half is preserved as an explicit ### Migration block rather than
dropped with the prose around it.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Qcu9ByJfuKBQxmrWb8rSrN

CHANGELOG.md

@@ -6,16 +6,6 @@ All notable changes to seismo-relay are documented here.
 
 ## Unreleased
 
-**Blastware Event/FFT-Report parity — the FFT, the USBM compliance chart, and
-the sensor self-check.** Three analyses Blastware derives from event data,
-reverse-engineered against BE12844 (MiniMate Plus) reports and reproduced in
-seismo-relay: the compliance chart and the sensor-check strip now render on
-the event-report PDF, and the FFT reproduces Blastware's FFT Report. All three
-are additive and read from data already on disk — the `.h5` samples and the
-retained raw BW binary — so there is **no `.h5`/DB change, no migration, and no
-backfill**: a report regenerated for an existing event simply gains the new
-panels.
-
 ### Added
 - **Rescue-on-connect for `bridges/ach_server.py`** — `--stop-monitoring`
   (SUB 0x97), `--disable-ach` (SUB 0x2C read → 0x7E write → 0x7F confirm) and
@@ -85,6 +75,16 @@ panels.
 
 ---
 
+### Migration
+
+**None.**  Every change here is additive and reads from data already on disk —
+the `.h5` samples and the retained raw BW binary.  No `.h5`/DB change, no
+schema change, no migration, no backfill, and **no `TOOL_VERSION` bump**: a
+report regenerated for an existing event simply gains the new panels, and the
+`ach_server` rescue flags don't touch the codec.
+
+---
+
 ## v0.30.0 — 2026-09-12
 
 **The series-4 correctness release** — the Thor / Micromate counterpart to