chore(release): v0.27.0 — decoder verified at scale; offset investigation

Bumps pyproject, TOOL_VERSION, README and CLAUDE.md to 0.27.0. sfm/server.py
now derives its version from TOOL_VERSION (c8c4ec2), so that constant is the
single source of truth for the service version and the sidecar stamp alike.

What ships:
  - histogram partial-final-block fix (4 files recovered, 0 regressed)
  - interval-based find_twins matching (terra-view #102 sub-task 2)
  - /health no longer reports a hard-coded 0.1.0
  - 793 NUL bytes stripped from CLAUDE.md (made grep skip it as binary)
  - docs/offset_investigation.md, and the offset detectors
  - scratch/verify_against_ascii.py

Verification: the series-3 codec now decodes 14,338 / 14,338 archive pairs
exactly against their Blastware ASCII exports (1,249 waveform + 13,089
histogram, 45 units, back to 2018) — 11x the ground truth the prod store
carried, and it supersedes the old "per-sample on 11% of files" caveat.

Independent check on the scale: 19,244 healthy channel-events sit at a
pre-trigger floor of exactly 0.000 (62.7%), 94.5% within one quantisation
unit, median +0.0000. No zero-point bias in the decoder.

⚠ TOOL_VERSION moved, so the next prod backfill regenerates the whole store
(~2 hours on the NAS). That is intended — it is what publishes the 4 recovered
histograms — but it is not a no-op; schedule it.

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

CHANGELOG.md

@@ -6,6 +6,16 @@ All notable changes to seismo-relay are documented here.
 
 ## [Unreleased]
 
+---
+
+## v0.27.0 — 2026-08-28
+
+**Per-sample decoder verification at scale, plus the offset investigation.**
+The series-3 codec is now verified sample-by-sample against **14,338** preserved
+Blastware ASCII exports — 1,249 waveform and 13,089 histogram, spanning 45 units
+and files back to 2018. That is 11x the ground truth the production store
+carried, and it found one real codec bug (below).
+
 ### Fixed
 - **Sub-minute histograms with a partial final block decoded to nothing**
   (`histogram_codec.detect_multi_interval_stride`). The stride search confirmed
@@ -43,6 +53,37 @@ All notable changes to seismo-relay are documented here.
   same-serial histogram). `window_seconds` is retained but ignored. Fixes terra-view #102
   sub-task 2.
 
+- **`/health` reported a hard-coded `0.1.0`** instead of the real service version.
+  `sfm/server.py` now derives its version from `minimateplus.event_file_io.TOOL_VERSION`,
+  making that constant the single source of truth for the service version and the
+  sidecar stamp alike — one place to bump at release.
+
+- **`CLAUDE.md` had 793 NUL bytes appended** after its last line, which made `grep`
+  treat the file as binary and silently skip it. Present since at least v0.21.0.
+  Stripped.
+
+### Added
+- **`docs/offset_investigation.md`** — a dated journal of the "offset" hardware
+  fault: base rate, detector design, per-unit case files, ruled-out hypotheses
+  (each kept with the evidence that killed it), and Instantel's own autozero
+  procedure with its 2027–2069 acceptance window.
+- **`scratch/verify_against_ascii.py`** — decodes a corpus of BW binaries and
+  diffs every sample against the paired `_ASCII.TXT`. Includes a saturation
+  carve-out: BW clamps clipped events to the range maximum and writes `OORANGE`,
+  while the decoder faithfully reports counts past nominal full scale.
+- **`scratch/offset_scan3.py`** — offset detector. Measures the resting floor in
+  the *pre-trigger* window (definitionally quiet) and requires it to hold across
+  pre / middle / end. Result: **5 of 45 units (11%)**, stable across a 2x
+  threshold range. Supersedes `offset_scan.py` and `offset_scan2.py`, both kept
+  as the reasoning trail.
+
+### Verified
+- **19,244 healthy channel-events sit at a pre-trigger floor of exactly 0.000
+  (62.7%), 94.5% within ±1 quantisation unit, median +0.0000.** No systematic
+  zero-point bias in the decoder — an independent confirmation of the
+  32000-count geo full scale, arrived at from a different direction than the
+  ASCII sample comparisons.
+
 ---
 
 ## v0.26.0 — 2026-08-27