feat: add C and C++ cross-reference parsing

[liumuqing]

First, thank you for building and maintaining this project. I really like code-index-mcp and have been using this MCP in my own workflow; it is genuinely useful for navigating and understanding codebases.

This PR adds specialized C/C++ parsing support to the deep index pipeline.

Summary:

• Add a tree-sitter based C/C++ parsing strategy.
• Parse .c and .h files as C, and .cpp/.cc/.cxx/.hpp/.hh/.hxx files as C++.
• Extract includes, functions, structs, unions, enums, C++ classes, namespaces, and methods.
• Record same-file and cross-file call relationships through the existing called_by / pending_calls flow.
• Categorize struct/union/enum/interface symbols in file summaries so they appear with class-like type symbols.
• Add strategy-level and SQLite deep-index tests for C/C++ symbols and cross-file calls.

Validation:

• uv run --with pytest pytest tests/strategies tests/indexing/test_c_cpp_called_by.py tests/indexing/test_sqlite_index_builder.py tests/indexing/test_javascript_called_by.py tests/indexing/test_typescript_called_by.py tests/indexing/test_rust_deep_index.py
• 25 passed

I also smoke-tested this against Linux kernel subtrees under /Volumes/Code/linux, including init and kernel/sched, and verified that C headers, structs, functions, and cross-file called_by relationships are populated.