getInterseqScale() was calling getCorrIncExact() which returns 0.5*corr_offset
for half_step mode instead of the actual integer keyframe step getCorrInc()
(= corr_offset/2 with integer division). For odd corr_offset (e.g. 5) with
half_step=true these differ: getCorrIncExact()=2.5 vs getCorrInc()=2.

This caused getHalfBeforeAfterPixXY() to compute:
  after - before = RSLT_VX * 0.5   (wrong; observed by user)
instead of:
  after - before = RSLT_VX * corr_inc/corr_offset = RSLT_VX * 0.4  (correct)

recalcOmegas() uses the same interseq_scale in the inverse direction, so it
also reconstructed RSLT_VX/VY incorrectly from neighbouring keyframe positions.

Fix: use getCorrInc() (integer) in getInterseqScale() so that BX/AX positions
correspond to actual raw-frame midpoints and the velocity round-trip is exact.

Also adds an unambiguous convention comment near RSLT_VX/RSLT_VY in
CuasMotionLMA.java documenting all required scale conversions.

The omega calculation (getFrameVelocityScale * ifov * fps_raw) is independent
of corr_inc and was already correct.
Co-authored-by: Claude <claude@elphel.com>