fix(series4): Thor/Micromate decoder is now per-sample exact

Verified against Thor's own CSV exports, which carry a per-sample
four-column block beside every binary (CSV/<name>.IDFW.csv). Those 1,012
paired files were in the corpus all along; the decoder had been pinned to
a superseded walker on the stated grounds that "Thor has no ASCII ground
truth in the corpus and its geo scaling is separately suspect". Both
premises were false.

  IDFW per-sample exact      39.1%  -> 100.000% (1,057,536/1,057,536)
  IDFW files fully exact     0/153  -> 153/153
  IDFW PPV median error      -3.32% -> -0.002%
  IDFH within 2% of Thor PPV 51.1%  -> 100.0% (858/858)
  prod IDFW, 8 units         -3.3%  -> -0.001%

Four independent root causes:

- Geo LSB was 0.0003, the 4-dp *display rounding* of the real
  0.000310308 mistaken for the LSB, so every series-4 geophone sample
  read 3.3% low. Pinned to +-6e-11 by intersecting 991,415 rounding
  constraints; corroborated by the +-full-scale seed (+-32226) left in
  unwritten IDFH slots. IDFH had a separate, also wrong, 10.0/32768.

- IDFH histograms were capped at 250 intervals: the segment validator
  required the interval counter's high byte to be zero, but the counter
  is a uint16 cumulative index, so every segment past interval 255 was
  rejected. Runs over ~4 hours lost their tail, often the peak.
  540/858 corpus files affected.

- Record mode 00 00 (raw int16, 10-byte header) was unhandled and fell
  through the dispatch, silently dropping each channel's first 512
  samples -- the long-standing "loud events truncate" symptom.
  MODE_ABSOLUTE is now also accepted as a segment-0 preamble.

- The body-offset search matched 00 02 00 *inside* record headers,
  selecting a candidate part-way down the chain and decoding a
  rotation-shifted body. It now anchors on record headers and takes the
  chain head (6 ms/file).

Also fixes the separately tracked "UM-series decodes ~1000x low" bug.
Series-3 re-verified unchanged at 14,338/14,338 exact after the shared
waveform_codec change.

Known open: 41/575 prod IDFW files (7%, mostly UM12947/UM20147) decode
with unequal channel lengths and also fail metadata extraction -- a
different header variant with no Thor export in the store.

NOTE: this is a codec change; the Thor store owes a regeneration via
scripts/backfill_thor_events.py.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Ru8Lg9HkkYvX9VWWo65SmL

CLAUDE.md

@@ -24,9 +24,23 @@ Read this first when picking the project back up.
   Independent corroboration of the 32000-count scale: 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 zero-point bias.
-- **Series-4 (Thor / Micromate) is NOT verified.** UM-series sits at ~48%
-  against device peaks with a ~1.7% systematic bias and a near-zero tail.
-  Thor IDFW is pinned to `decode_waveform_legacy` deliberately.
+- **Series-4 (Thor / Micromate) is now verified per-sample (2026-09-10).**
+  **1,057,536 / 1,057,536** geo samples across all 153 genuine Thor waveform
+  files reproduce Thor's own CSV export exactly; IDFH peaks are within 2% on
+  858/858 (median -0.004%).  The ground truth was in the corpus all along —
+  Thor writes `CSV/<name>.IDFW.csv` beside each binary with a **per-sample**
+  four-column block.  Harness: `scratch/verify_thor_against_csv.py`.
+  Four bugs, all fixed: geo LSB was `0.0003` (display rounding of the real
+  `0.000310308`, so every sample read **3.3% low**); the IDFH segment
+  validator required a zero counter high byte, **capping every histogram at
+  250 intervals**; record mode `00 00` (raw int16) was unhandled, silently
+  dropping each channel's first 512 samples; and the body-offset search
+  matched `00 02 00` *inside* record headers, decoding a rotation-shifted
+  body.  IDFW is no longer pinned to `decode_waveform_legacy`.
+  Series-3 re-verified unchanged at 14,338/14,338 after the shared-codec
+  change.  **Still open:** 41/575 prod IDFW files (7%, mostly UM12947) decode
+  with unequal channel lengths and have no Thor export — pull their CSVs
+  before touching it.
 - **Open, not blocking:** 14 sensitive-range files show an exact 8x
   (= 10.0/1.25) units discrepancy; `scripts/backfill_sidecars.py --force` also
   inserts DB rows for store files that have none (one-time per store) and the
@@ -120,20 +134,34 @@ should not import from `sfm/`, must not touch a DB, and have no I/O
 beyond reading files passed as arguments.  Keep them pure — both
 tiers can then depend on them without circularity.
 
-#### Thor IDF binary codec (2026-05-28)
+#### Thor IDF binary codec (updated 2026-09-10)
 
 `micromate/idf_file.read_idf_file()` decodes both Thor IDFW
-(waveform) and IDFH (histogram) binaries.
+(waveform) and IDFH (histogram) binaries.  **Verified per-sample
+against Thor's own CSV exports** — see
+`scratch/verify_thor_against_csv.py`.
 
-- **IDFW** reuses `decode_waveform_v2()` on the body at fixed file
-  offset `0x0f1f`.  Sample fidelity is 87–99% byte-exact on quiet
-  events; loud events hit the BW codec's known walker-stops-early
-  limitation.
-- **IDFH** has its own segment-based decoder: `[len_be][0a 00 00 00]
-  [00 NN][05 3f]` + N × 72-byte interval records (4 × 16-byte
-  per-channel min/max/halfp).  All 859 Thor IDFH corpus files
-  decode (181,071 intervals); peak matches sidecar within ~1.8%
-  (ADC quantization).
+- **IDFW** uses the series-3 record-chain `decode_waveform_v2()`.  The
+  body offset is **not** fixed: it is `<chain-head record> + 7`, found
+  by `_find_waveform_body_offset()` anchoring on record headers.  All
+  **153/153** genuine Thor waveform files decode per-sample exact
+  (1,057,536/1,057,536 samples).
+- **IDFH** segment header is `[len_be][0a 00 00 00][counter_be][05 3f]`,
+  where `counter` is a **uint16 cumulative interval index** — it must
+  not be constrained to a zero high byte (that capped histograms at 250
+  intervals).  Intervals whose `min > max` on all channels are unwritten
+  slots carrying a ±full-scale seed and are skipped.  858/858 files land
+  within 2% of Thor's PPV (median -0.004%).
+- **Geo LSB is `0.000310308` in/s per count** (full scale 10.0 in/s =
+  32226.05 counts).  Series-3's 32000-count scale does NOT apply.
+- **Record modes** are `02 00` deltas (14 B header), `01 00` absolute,
+  `00 03` raw 12-bit, and `00 00` **raw int16** (all 10 B headers).
+  `01 00` and `00 00` are also valid as the implicit segment-0 preamble.
+
+⚠ **Thor's histogram PPV has a 0.0050 in/s display floor.**  41.4% of
+prod IDFH sidecars report a component PPV exceeding their own vector
+sum — impossible.  On quiet files our decode is *more* accurate than
+the reference; do not "fix" the decoder to match it.
 
 The two outlier `BE9439_*` files in the Thor example corpus are
 actually Series III Blastware binaries that share the `.IDFW`/`.IDFH`
@@ -399,15 +427,19 @@ with zero mismatches.  Before: 1 of 1196.
   `BE12599/N599LPWJ.980W` @849, `BE9558/K558LOF2.820W` @1485.
 (The series-3 histogram codec was fixed 2026-08-25 — see below.)
 
-- **Micromate (UM-series) IDF decode is ~1000× low** — e.g.
-  `UM11402_20260406130113.IDFW` gives a Tran peak of 0.0009 in/s against
-  a device-reported 1.1168.  The Thor IDF path decodes sanely, so this
-  is UM-specific.
-- **Thor IDF per-count LSB** — after the 32000 geo full-scale
-  correction, series-4 Thor peaks sit at a median 0.983 of the
-  device-reported peak (was 0.960 under 32768).  Closer but not exact;
-  Thor likely uses its own per-count LSB rather than the BW
-  16-count/0.005 in/s convention.
+- ~~**Micromate (UM-series) IDF decode is ~1000× low**~~ — FIXED 2026-09-10.
+  `UM11402_20260406130113.IDFW` now decodes Tran 1.1168 / Vert 4.3220 /
+  Long 0.9135, matching the device report exactly.  Root cause was the
+  body-offset search landing inside a record header plus the unhandled
+  `00 00` record mode, not anything UM-specific.
+- ~~**Thor IDF per-count LSB**~~ — RESOLVED 2026-09-10.  The 0.983 ratio was
+  exactly `0.0003 / 0.000310308`.  Thor's geo LSB is **0.000310308 in/s per
+  count** (full scale 10.0 in/s = 32226.05 counts), pinned to ±6e-11 by
+  intersecting 991,415 rounding constraints from Thor's own exports and
+  corroborated by the ±full-scale seed (`±32226`) left in unwritten IDFH
+  interval slots.  Series-3's 32000-count scale does **not** carry over.
+  Note `10.0/32226` is very slightly wrong — see
+  `docs/idf_protocol_reference.md`.
 
 ### Decoded sample counts (across the fixture bundle)