fix(histogram): block is big-endian + terminal block tail — 1/1196 to 1211/1211

Two errors in the series-3 histogram block model, both found by diffing
against the per-interval data table in the preserved Blastware ASCII
exports (1211 files in the prod snapshot — far stronger ground truth
than the header PPV used previously).

1. The block is uniformly BIG-ENDIAN.  Peaks and half-periods are uint16
   BE (T_peak [5:7], T_halfperiod [7:9], V_peak [9:11], V_halfperiod
   [11:13], L_peak [13:15], L_halfperiod [15:17], M_peak [17:19],
   M_halfperiod [19:21]); only block_ctr [2:4] is little-endian.

   The old uint8-peak model silently CLIPPED any peak above 1.275 in/s:
   the final interval of BE18193/T193LQ9K.OE0H reads 8.270 in/s in BW's
   export (1654 counts = 0x0676) and decoded as 0x76 = 118 = 0.590.

   The byte documented as a per-channel "annotation" was never an
   annotation — it is the half-period's high byte, which is exactly why
   it was non-zero on the sub-Hz intervals BW renders as "<1.0".

   The marker is block[4] alone.  Testing [4:6] as a uint16 LE marker
   forced block[5] == 0, which is what capped the peak at one byte.

2. The final block of each stream carries tail 9c 06 00 42 instead of
   1e 0a 00 00, and holds arbitrary bytes at [21:23].  Rejecting it
   dropped the last interval of nearly every histogram — frequently the
   interval holding the event peak, so the file's PPV read low.

Verified end to end through the production path: 1211/1211 histograms
decode exactly (interval count + every per-interval peak), plus 842,442
per-interval frequency comparisons with zero mismatches.  Previously
1 of 1196 files was fully correct.

decode_histogram_body_full records expose `is_terminal` in place of the
removed `annotations` tuple.  +6 tests.  No regressions: full-suite
failure list unchanged from baseline.

NOTE: stored histogram .h5 files need regenerating to pick this up.

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

CHANGELOG.md

@@ -8,6 +8,36 @@ All notable changes to seismo-relay are documented here.
 
 ### Fixed
 
+- **Series-3 histogram block is uniformly big-endian, and the stream's final
+  block has its own tail — the codec was clipping large peaks and dropping the
+  last interval of nearly every histogram.**
+
+  - **Peaks and half-periods are `uint16` big-endian**, not `uint8` plus an
+    "annotation" byte: `T_peak` `[5:7]`, `T_halfperiod` `[7:9]`, `V_peak`
+    `[9:11]`, and so on.  Only `block_ctr` `[2:4]` is little-endian.  The old
+    model silently **clipped any peak above 1.275 in/s** — the final interval
+    of `BE18193/T193LQ9K.OE0H` reads 8.270 in/s in Blastware's own export and
+    decoded as 0.590.  The "annotation" byte was the half-period's high byte,
+    which is why it was non-zero exactly on the sub-Hz intervals BW renders
+    as `<1.0`.
+  - **The marker is `block[4]` alone.**  Testing `[4:6]` as a `uint16 LE`
+    marker forced `block[5] == 0` — which is what capped the peak at one byte
+    in the first place.
+  - **The last block of each stream carries tail `9c 06 00 42`** instead of
+    `1e 0a 00 00`, with arbitrary bytes at `[21:23]`.  Rejecting it dropped
+    the final interval of nearly every histogram, and that interval is
+    frequently the one holding the event peak — so the file's reported PPV
+    came out low.
+
+  Verified against **1211 production histograms** paired with their preserved
+  Blastware ASCII exports, which carry a full per-interval data table:
+  **1211/1211 now decode exactly** (interval count plus every per-interval
+  peak), and 842,442 per-interval frequency comparisons match with zero
+  mismatches.  Before this fix: **1 of 1196**.
+
+  `decode_histogram_body_full` records now expose `is_terminal` in place of
+  the removed `annotations` tuple.
+
 - **Geophone full scale is 32000 ADC counts, not 32768 — every geo reading was
   2.3% low.** The verified body codec emits geo samples in 16-count units whose
   documented LSB is exactly 0.005 in/s, and `decoded_to_adc_counts` multiplies