test(p10-1d-followup): add in-file unit tests to C AST extractor
Mirrors the cpp.rs 15-test pattern. Covers function_definition (incl.
pointer-return, static/extern/inline), struct_specifier / enum_specifier /
union_specifier (named), anonymous struct/enum/union → glue, typedef-wrapped
struct → glue (per spec risks note), preprocessor directives → glue, empty
file → <module> post-pass, preprocessor-only → <module>, mixed fn + glue →
<top-level> present, determinism (20 runs). 17 tests total.
Reviewer nit #1 (PR #156 code-reviewer).
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
altair823
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