Finite-diff vel1 was dominated by single-frame pose glitches from
algorithm failures (low altitude / dense canopy), giving 600+ deg/s
for sequences where _dt shows ~10 deg/s.

Changes:
- parse _dt entries (algorithm's smooth polynomial derivatives) as
  primary velocity metric — unaffected by pose glitches
- keep finite-diff as secondary; glitch = fd_max / dt_max
  (~1 = clean data, >>1 = pose jumps / algorithm failure)
- --max_glitch filter hides suspect sequences from console output
  (TSV always contains all rows for Calc analysis)
- removed windowed finite-diff (vel5) — _dt makes it redundant
Co-authored-by: Claude <claude@elphel.com>