Moves the four structural validators for INTEROP file formats
(--validate-stl, --validate-png, --validate-blp, --validate-jsondbc)
out of main.cpp into a new cli_validate_interop.{hpp,cpp} module.
These check files coming in/out of wowee from third-party tools,
distinct from cli_format_validate.cpp which validates the native
open formats (WOM, WOB, WOC, WHM).
main.cpp shrinks by 524 lines (10,644 to 10,121). Each validator
preserves its --json output mode for machine-readable reports.
