--ego (egomotion.csv) was required but is not always present (it is an
optional output file).  --xml (*-INTERFRAME.corr-xml) is always written
by the pipeline and contains the same x/y/z/a/t/r values.

Changes:
- add parse_interframe_xml(): sorts timestamps → same frame order as COLMAP
- --xml is the new preferred input; --ego is optional and overrides --xml
  (it additionally provides IMS/PIMU columns)
- at least one of --xml or --ego is required
- IMS/PIMU [SKIP] messages suppressed when using XML (NaN is expected)
- verified: XML and egomotion.csv produce identical results
Co-authored-by: Claude <claude@elphel.com>