Reject incompatible RFC 8785 generation options

[ydah]

Reject nonempty whitespace options and enabled ascii_only, script_safe, or allow_nan with ArgumentError when rfc8785 is enabled.

JSON.generate("é", rfc8785: true, ascii_only: true)
# Before: '"\u00e9"'
# After: ArgumentError

JSON.generate([Float::NAN, Float::INFINITY], rfc8785: true, allow_nan: true)
# Before: '[NaN,Infinity]'
# After: ArgumentError

Validate configuration and generation, including options changed through State setters. JSON.dump defaults allow_nan to false when rfc8785 is enabled.

JSON::Fragment remains supported for reusing canonicalized fragments. Its contents are inserted as is; callers must ensure they are canonical.

Addresses the incompatible-options TODO in #1091.