bench+docs: IG threshold LSB is FS/256; factory HA* seed values recovered

The captured pulse headers carry the factory's IG programming: hunts
off, travels abort-only (HAar=133 at +/-4 g), the cut alert-only
(HAxr=132, HAyr=112), Z and idle never armed. The factory abort value
itself rules out an FS/128 LSB (4.16 g would sit over the measurable
range), and two Z-only bench windows bracket gravity between
thresholds 100 and 150, which only FS/256 explains. The drill script's
printed g conversion is corrected, and the stale motion-profile
reading of HAxr/HAyr/HAar as accel limits is corrected in the facts
bank.

No catalog consequence: a bench script's printed conversion and the
docs record; no shipped behavior changes.

scripts/bench/accel_crash_probe.py

@@ -41,8 +41,9 @@ Usage:
             but liveness is down meanwhile, so stop the controller and
             forgectrl first)
   --ths N   per-axis threshold register value 0..255 (default 40); the
-            LSB is full-scale dependent, so the printed CTRL4 FS fixes
-            the g conversion (+/-2 g default: ~15.6 mg/LSB, 1 g ~= 64)
+            LSB is full scale / 256 (bench-confirmed against gravity:
+            +/-2 g gives ~7.8 mg/LSB, 1 g ~= 128), so the printed CTRL4
+            FS fixes the g conversion
   --dur N   IG_DUR1 duration counter, samples at the running ODR
             (default 0 = fire on the first over-threshold sample)
   --axes    which axes arm high events (default xyz)
@@ -172,7 +173,7 @@ def main():
     if (ctrl1 & 0x70) == 0:      # ODR bits 6:4 zero = power-down; IG needs
         wr(fd, CTRL1, 0x6F)      # a running ODR: 800 Hz, BDU, XYZ on
         ctrl1 = rd(fd, CTRL1)
-    lsb_mg = (fs * 1000.0 / 128.0) if isinstance(fs, int) else 0
+    lsb_mg = (fs * 1000.0 / 256.0) if isinstance(fs, int) else 0
     print('WHO_AM_I=0x%02x CTRL1=0x%02x CTRL4=0x%02x  FS=+/-%sg  '
           'ths=%d (~%.0f mg, ~%.2f g)  dur=%d  axes=%s  mode=%s'
           % (who, ctrl1, ctrl4, fs, ths, ths * lsb_mg, ths * lsb_mg / 1000.0,