fix(histogram): three defects found by a full series-3 sweep — 11603/11603 clean

Swept every series-3 binary with the live decoder against five
independent checks: decode exceptions, zero samples, unequal geo channel
lengths, peaks above range full scale, decoded peak vs device-reported
PPV, and waveform length vs declared record time.

1. block[22] is NOT a constant and must not be tested.  Documented as
   always 0x00, it carries data on loud blocks, and rejecting those threw
   away the interval holding the event peak.  BE18350/T350L7HR.NL0H
   block 92 has block[22]=0x26 and a Tran peak of 0x0563 = 1379 counts =
   6.895 in/s — exactly the device-reported PPV — while the file decoded
   to 0.015 in/s.  block[0]==0, block[4]==0x0A and the 4-byte tail are
   six bytes of constraint, which is what keeps trailer content out.

2. Block-model dispatch now goes on signature strength rather than on
   whichever decoder returns first.  A multi-interval body also yields
   scattered standard-tail blocks by coincidence, so "first non-empty"
   handed 193 BE18193 files to the standard walker and produced peaks of
   149 in/s against a 10 in/s full scale.

3. Multi-interval stride detection requires the block counter to
   increment by exactly 1.  Without it the detector false-positives on
   ordinary standard-block bodies: they carry a header every 32 bytes,
   and 192 = 12 + 20*9 and 512 = 12 + 20*25 are both multiples of 32, so
   a stride "fits" while skipping 6 or 16 real blocks.  That misrouted
   9,082 files.

Partial-block garbage is trimmed within the final block only, stopping
at the first slot with a non-zero tail word or a geo peak above full
scale (2000 counts in 16-count units).  Trimming purely from the end
left garbage stranded behind a slot that happened to have a zero tail
word; trimming on the tail word alone truncated four BE9440 files by up
to 2,800 intervals.

Result: 11,603 / 11,603 series-3 binaries clean on every check.
Ground truth unchanged: 1211/1211 histograms exact per-interval, 75/75
waveform sample counts exact (73/75 fully exact, the 2 differ by 1 LSB
on rail samples), and the multi-interval fixture still matches its BW
ASCII export on all 45,680 values.

Tests: 259 passed, failure list unchanged from baseline.

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

tests/test_histogram_codec.py

@@ -457,11 +457,22 @@ def test_terminal_block_exempt_from_byte22_constraint():
     assert ch is not None and len(ch["Tran"]) == 2
 
 
-def test_standard_block_still_requires_byte22_zero():
-    """The [22] == 0x00 constraint is what keeps trailer content out, so it
-    must still apply to standard-tail blocks."""
-    ch = decode_histogram_body(_mk_block(b22=0x01))
-    assert ch is None
+def test_standard_block_accepts_nonzero_byte22():
+    """block[22] is NOT a constant and must not be tested.
+
+    It was documented as always 0x00, but it carries data on loud blocks.
+    Rejecting those threw away the interval holding the event peak:
+    BE18350/T350L7HR.NL0H block 92 has block[22]=0x26 and a Tran peak of
+    0x0563 = 1379 counts = 6.895 in/s — exactly the device-reported PPV —
+    while the file as a whole decoded to 0.015 in/s.
+
+    block[0]==0x00, block[4]==0x0A and the 4-byte tail are six bytes of
+    constraint, which is what keeps trailer content out.
+    """
+    ch = decode_histogram_body(_mk_block(t_peak=1379, b22=0x26))
+    assert ch is not None
+    assert ch["Tran"] == [1379]
+    assert geo_count_to_ins(ch["Tran"][0]) == pytest.approx(6.895)
 
 
 def test_marker_is_single_byte_not_uint16():