The `rdoc -C` coverage report previously displayed undocumented items
using Ruby syntax (`class ... end`, `def ...; end`). For C source files
like Ruby's object.c, this made the output appear as if the C file was
being treated as a Ruby script.
Replace with a file-centric listing that groups undocumented items by
source file and type:
    object.c:
      Class:
        Refinement
      Method:
        Module#name object.c:135
          Undocumented params: mod.name->stringornil
- Group items by source file, sorted alphabetically
- Within each file, group by type (Class, Module, Constant, Attribute, Method)
- Sort items by line number within each type group
- Use ClassName#method / ClassName.method notation
- Show clickable file:line references, column-aligned
- Return plain strings from report/summary instead of Markup objects