v0.21.3

Docs: physics_machine_resume_from_line and physics_machine_preview_resume now describe resuming a job that was interrupted by the browser going away — a closed tab, a sleeping laptop, a power cut.

Mesh checkpoints the program and the reached line as it streams, so the job is offered back on the next load. That kind of resume is flagged restored: the controller has been round a power cycle, so it is not homed, its tool is unknown, and the work must still be clamped exactly where it was. Home, confirm the work origin and re-zero Z before taking one.