feat: add C/C++ language support

[kapelner]

Summary

Adds C/C++ language support to graft's Tier-1 extraction, following the same pattern established for the other supported languages (Go, Python, TS). One "cpp" grammar (tree-sitter-cpp) parses the whole C/C++ family — .c/.h/.cpp/.hpp/.cc/.cxx/.hh/.hxx — uniformly, the same approach clangd and most polyglot tooling take. No separate .c-only language for v1 (documented limitation below).

• Classes/structs/enums/templates extracted directly (class_specifier/struct_specifier/enum_specifier all carry a name field; template_declaration spans are attributed back to the templated class/function so the card doesn't cut off the template<typename T> line).
• Functions/methods need a dedicated describeCpp(): function_definition carries no name field at all — it's buried in a declarator chain that can be a plain identifier, an out-of-class Class::method qualifier, a destructor (~Class), or an operator overload (operator==). A header-declared prototype and its .cpp out-of-line definition resolve to exactly one node, not two or zero — the single most important case to get right for a header/source-split codebase.
• Visibility is stateful: an access_specifier token (public:/private:/protected:) applies to every subsequent class member until the next one (default private for class, public for struct) — unlike every other supported language's per-node exported check, which needed new plumbing (ctx.cppAccess threaded through the walk).
• Heritage (: public Base) always emits extends — C++ has no interface keyword, so no "first = extends, rest = implements" heuristic is needed (simpler than the analogous C#-style precedent this was modeled on).
• Calls: member (./->), qualified (Class::static_method, ns::fn) resolved via a direct recvType supply (bypassing the bindings-lookup path, since the qualifier already IS the type name).
• #include edges capture both "local.h" and <system> forms.

Known limitations (documented in code + CHANGELOG)

• Macros that expand to declarations aren't understood — same class of limitation as any AST-only tool.
• Template specializations are treated as ordinary functions/methods by name (no specialization identity modeled).
• A bareword #include "local.h" (no ./ prefix) only resolves to an in-repo file when it happens to match a real repo-relative path — C's directory-relative-without-prefix include convention isn't specially handled by resolve.ts's existing import resolver.

Test plan

• tsc --noEmit clean
• npm run build clean
• test/graph-cpp.test.ts — 7 tests covering classes/structs/enums/templates, the header/source-split single-node guarantee, access-specifier visibility, heritage, member/qualified calls, #include edges, and contains edges
• Full existing test suite green, no regressions
• Smoke-tested via graft build against a real header/source-split C++ fixture

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