fix(cpp): recover callables after preprocessor-gated signatures

[blankanswer]

Summary

Fixes #946.

C++ files that gate alternate out-of-line method signatures with #ifdef / #else / #endif can confuse the raw tree-sitter parse and cause methods after the conditional block to disappear from the graph.

This PR extends the existing C/C++/CUDA preprocessor second pass so it can recover missing Function / Method definitions from the expanded AST, while keeping the normal raw-source extraction as the primary source of truth.

What Changed

• Added a regression test for the SurfaceFlinger-style conditional signature shape.
• Added a lightweight definition-walker entry point that does not emit a Module node or variables.
• During the existing preprocessor second pass, merge only missing callable definitions from the expanded AST.
• Deduplicate recovered callables by label + qualified_name.
• Did not change vendored tree-sitter grammars.

Validation

• make -f Makefile.cbm SANITIZE= build/c/test-runner build/c/codebase-memory-mcp
• build/c/test-runner extraction — 211 passed
• build/c/test-runner c_lsp — 750 passed
• git diff --check
• CLI smoke test with a temporary C++ repo:
  • index_repository
  • search_graph verified both commit and composite are indexed after the #ifdef/#else/#endif signature block.

[DeusData]

Thanks for the focused C++ parser fix. Triage: parser recovery bug for #946, normal priority. Review focus is ownership of recovered callables, avoiding duplicate definitions across #ifdef branches, and keeping unrelated extraction behavior unchanged.