Behaviour-preserving move of the --load-assemble-list JSON parser and
its plate/object structs from the CLI into libslic3r/Format/AssembleList,
so the format can be unit tested. The parser returns its own
AssembleListResult and takes the plate limit as a parameter; CLI::run
maps the result to the same exit codes as before. Every validation rule
and log message is unchanged.
Adds Catch2 coverage of the valid layout and each validation rule.
