fix(codec): geo full scale is 32000 counts; 4 walker framing cases; channel-id from header

Two independent bugs, both found by diffing 75 production events against
their preserved Blastware ASCII exports (<store>/<serial>/<file>_ASCII.TXT).

1. Geo full scale was wrong — every geophone reading was 2.34% low.
   The codec emits geo samples in 16-count units with a documented LSB of
   exactly 0.005 in/s, and decoded_to_adc_counts multiplies by 16, so one
   ADC count is 0.005/16 in/s and 10.000 in/s is 10.0/(0.005/16) = 32000
   counts.  sfm/event_hdf5.py and minimateplus/event_file_io.py both
   divided by 32768 (2^15), scaling every sample and derived peak down by
   1 - 32000/32768.  The error scales with amplitude, so it was invisible
   on quiet events and worst on the loud ones that matter for compliance.
   Mic is unaffected (it back-solves its scale from the device peak).

   216 per-channel comparisons: 32768 -> 151/216 exact, worst error 0.238
   in/s on a 10 in/s event; 32000 -> 216/216 exact, worst 0.005 = 1 LSB.

2. walk_body silently truncated channels on four unhandled framing cases.
   An unrecognised tag ends the walk and decode_waveform_v2 returns
   whatever it got, so this surfaced as short channels, never an error:
     - wide-NN RLE `0X NN` (runs longer than 252 samples)
     - `30 NN` with NN > 0x10 (the old cap was arbitrary)
     - variable-width `40 NN` headers: NN counts previous-channel
       continuation deltas, so the header is 2*NN + 16 bytes; `40 01`
       and `40 03` occur alongside `40 02`
     - tagless segment headers: no `40 NN` tag at all, just the 14-byte
       tail [field2:2][len:2][channel_id:4][marker:2][anchors:4]

Also: the header field documented as a "monotonic uint32 LE counter" is
really [channel_id][00][00][segment_index], with 0x46=Tran 0x47=Vert
0x48=Long 0x49=MicL — verified on 1697/1697 segment headers, zero
disagreements.  decode_waveform_v2 now takes the channel from that field
instead of rotation position, which was fragile: one missed header
desynced every channel after it.

parse_segment_header now returns n_prev_deltas/prev_deltas/marker/
anchors/channel/segment_index; the old fixed_pattern (02 00 00 01)
conflated the 2-byte marker with the first anchor.

Ground-truth corpus, end to end through the production path:
  exact 37 -> 72, truncated 23 -> 3, full-length value errors 15 -> 0.
Store-wide, 729 of 1388 series-3 waveform events decode differently and
728 gain samples; the scale fix changes float values on all of them, so
stored .h5 files need regenerating.

Still open: 3 events truncate at a header variant with a variable-width
prefix (2/4/6 bytes) before the channel id and an `01 00` marker.
Documented in docs/instantel_protocol_reference.md with byte offsets.

+20 tests.  No regressions: the byte-exact fixture suite still passes and
the full-suite failure list is unchanged from baseline (16 pre-existing
failures from gitignored fixtures).

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

CLAUDE.md

@@ -264,6 +264,14 @@ then `decoded_to_adc_counts()` to scale to int16 ADC counts (geos × 16;
 mic pass-through).  The `.h5` sidecars SFM produces now contain
 correct samples for any event without walker edge cases.
 
+**Geo full scale is 32000 ADC counts, NOT 32768** (fixed 2026-08-25).
+One decoder unit = 16 ADC counts = exactly 0.005 in/s, so
+`10.000 in/s / (0.005/16)` = 32000.  Consumers must use
+`sfm.event_hdf5._GEO_INT16_FS` / `event_file_io._GEO_INT16_FS` (both
+32000).  Dividing by 32768 reads every geophone sample 2.34% low —
+that was a live bug in both modules until 2026-08-25.  Mic is
+unaffected (it back-solves its scale from the device-reported peak).
+
 The original int16 LE decoder is preserved as
 `_decode_a5_waveform_LEGACY` for reference but is not called.