_subtomo_angle_plan() used an explicit start_angle as the origin of a new
360-degree sweep. Resuming subtomo 2 at 85 deg generated 435..85; the >360
artifacts were silently skipped by _tomo_scan_at_angle's range gate, so the
resume began at 355 deg, re-measured done angles, never reached the rest and
reported wrong progress indices.
start_angle is now the literal resume point on the sub-tomogram's own grid:
the plan returns the remaining tail of the normal sweep plus a subtomo_offset
used for progress numbering (mirrors Flomni). Off-grid angles raise ValueError.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>