feat: replace regex ino prototype generation with clang AST parsing

[zackees]

Context

fbuild currently generates function prototypes for .ino preprocessing with regex-based extraction.

That is brittle for real C++ syntax, including templates, attributes, namespaces, multiline signatures, default arguments, comments, macros, references, qualifiers, and other common sketch patterns.

Relevant area:

• crates/fbuild-build/src/source_scanner.rs

Proposal

Replace regex-based .ino function prototype generation with a clang AST-based implementation, or an equivalent parser-backed approach that is robust against normal C++ syntax.

The implementation should preserve Arduino/PlatformIO compatibility while avoiding accidental prototypes from control-flow statements, macros, lambdas, class methods, or invalid parse fragments.

Acceptance criteria

• Prototype generation no longer depends on brittle regex matching.
• Tests cover common Arduino sketch functions and complex C++ signatures.
• Tests cover cases that should not produce prototypes, including lambdas, class methods, macros, control-flow blocks, and namespaced definitions.
• Generated prototypes are stable across repeated runs.
• Any clang dependency/runtime requirement is documented.

Open questions

• Should this use clang directly, libclang bindings, clang tooling JSON output, or a smaller parser-backed compatibility layer?
• How should parsing work before generated prototypes exist, given Arduino-style sketches may not be valid C++ yet?