feat(cli): validate command using loader + Parse_With_Schema

[berkeleynerd]

Implement mud-area validate --schema <schema.sml> --world <world.sml>.

Behavior

• Load schema via SML.Schema.Loader.
• Parse world via SML.DOM.Parser.Parse_With_Schema.
• Print clear validation result; exit 0 on success, non-zero with message/path on failure.