`.c` files were parsed with tree-sitter-cpp, so valid C that uses C++
keywords as identifiers (`int delete = 0; int new = 1;`, a parameter
named `class`) produced grammar-error diagnostics and lost symbols.
`.c` files now parse with tree-sitter-c (pinned =0.24.2, backend
`c:0.24.2`) while staying on the shared C/C++ extraction path, so C
sources and the headers they include still share one include/declaration
index. Headers (`.h`) keep the C++ grammar, which parses the C subset.
Test: cpp_xtu::c_sources_parse_with_the_c_grammar fails before this
change (grammar errors in util.c).
Use pinned grammar frontends and scoped heuristic resolution to recover declarations and prevent false dependencies. Constrain filesystem traversal and normalize Fold root aliases.
Add exact multilingual fixtures, seventeen regression tests, and a three-repeat benchmark to the quality gate.
