[Minor] Move HTML shingles implementation to separate C++ file

[Minor] Move HTML shingles implementation to separate C++ file

The HTML shingles code requires C++ (html_content, std::variant, etc.)
but was placed in #ifdef __cplusplus block in shingles.c (a C file),
causing linker errors.

Solution: Move all HTML-specific code to shingles_html.cxx which is
compiled as C++ and properly exports symbols with extern "C" linkage.

Files:
- shingles.c: Keep only C code (text/image shingles)
- shingles_html.cxx: New file with HTML shingles implementation
- CMakeLists.txt: Add shingles_html.cxx to build