Replaces the .nstdot nest-template mechanism for new-nest plate
defaults. NestDefaults persists units, plate size, quadrant, part
spacing, and edge spacing to a single JSON file (DefaultPath:
%APPDATA%/OpenNest/defaults.json). Load never throws: a missing,
corrupt, or partially valid file degrades field-by-field to the
built-in fallback values, and unknown fields or a future version
number are ignored.