Add cbindgen infrastructure to auto-generate C/C++ header from Rust extern
"C" surface at build time.
- Add cbindgen.toml config (C language, include guard, pragma_once, cpp_compat)
- Add build.rs to generate include/pdftract.h during cargo build
- Generated header compiles cleanly with gcc (C) and g++ (C++)
The header is the contract between libpdftract and C/C++ consumers.
Future extern "C" functions will automatically appear in the header.
Refs: pdftract-5rl5o
