feat(code-graph): add preliminary C++ language support

What does this MR do and why?

Part of #337 (closed). Adds C++ to the code-graph pipeline. C++ joins the CFamily language family (!1210 (merged)), sharing a CodeGraph with C so that #include-based cross-language resolution works out of the box.

What C++ can parse

  Namespaces         namespace math { ... }
  Classes/Structs    class Vec3 { ... };  struct Point { ... };
  Enums/Unions       enum Color { RED, GREEN };  union Data { ... };
  Typedefs           typedef struct { ... } Name;
  Templates          template<typename T> class Container { ... };
  Functions          int add(int a, int b) { ... }
  Member calls       obj.method()  obj->method()
  Qualified calls    Ns::func()  Class::static_method()
  Includes           #include "header.h"  #include <iostream>
  Bindings           int x = foo();  parameters;  assignment
  Control flow       if/switch branches, for/while/do/for-range loops

How resolution works

  main.cpp ──#include──► math/vec.hpp ──paired──► math/vec.cpp
     │                       │                        │
     │                   declarations              definitions
     │                   (Vec3, cross)             (Vec3::length, cross)
     │                       │                        │
     ▼                       ▼                        ▼
  ┌──────────────────────────────────────────────────────┐
  │              Shared CFamily CodeGraph                │
  │                                                      │
  │  main.cpp calls cross() ──resolves──► math/vec.cpp   │
  └──────────────────────────────────────────────────────┘

The IncludeGraph import strategy does BFS through transitive #include chains. For each reachable header (.h, .hpp, .hh, .hxx), it also searches the paired source file (.c, .cc, .cpp, .cxx). This means main.cpp including vec.hpp can resolve symbols defined in vec.cpp.

file_scope uses the filename (without extension) as the root scope for all top-level definitions, since C/C++ have no module/package system. A function add in math.cpp gets FQN math::add.

---

Integration tests

cpp/resolution.yaml -- namespaces, classes, includes, cross-file FQNs, enum constants, file discovery

cpp/hardcore.yaml -- negative assertions (no edges to static functions from other files, no edges to unincluded files), exact definition counts, templates, static function visibility, nested namespace scoping (outer::inner::Widget), duplicate edge checks, file containment assertions

Changelog

• langs/generic/cpp.rs -- C++ DSL handler (scopes, refs, imports, chains, bindings, branches, loops)
• linker/imports.rs -- IncludeGraph BFS strategy with header-to-source pairing
• linker/rules.rs -- IncludeGraph enum variant
• dsl/types.rs + engine.rs -- file_scope trait method
• config/lang.rs -- Cpp variant mapped to CFamily, extensions cpp/cc/cxx/hpp/hh/hxx
• treesitter-visit/ -- tree-sitter-cpp 0.23.4, SupportLang::Cpp