[Rust][FFI] Add low-level declarations for the public C header

[utilForever]

Parent: #137

Goal

Expose the stable C ABI accurately in the sys crate with the least maintenance burden.

Scope

• Choose committed declarations or generation from the installed header based on reproducibility
• Match calling convention, integer widths, and opaque types
• Keep generated output reviewable if generation is used

Non-goals

• Binding C++ headers
• Hand-maintaining duplicate semantic APIs
• Exposing internal symbols

Acceptance criteria

• Rust declarations match C sizes and function signatures.
• The sys crate links and calls the ABI version query.
• Invalid layout assumptions fail in tests.

Verification

Run cross-language size assertions and a sys-level create/destroy smoke test.