Add `-Winclude-shadow` to warn about multiple candidates for `INCLUDE`/`INCBIN`/`READFILE`

`rgbasm -I/--include` can specify multiple possible include paths, and the same file might exist in more than one. We have an unambiguous resolution sequence, but that doesn't mean users always intentionally expect it.

Start with rgbasm's -I/--include handling and the resolution paths for INCLUDE, INCBIN, and READFILE. Review the existing rgbasm tests, then consider the work complete when the new -Winclude-shadow warning consistently identifies cases where multiple include paths contain a candidate file.

Written by the indexing model from the issue text.