Struct typedef is not generated

Nobody has claimed this yet.

After #24, I was able to use cheadergen with CO2 (thanks!), and immediately noticed this:
```
#[repr(C)]
struct Foo {
x: i32,
y: i32,
}
```
Expected output:
```
typedef struct Foo {
int32_t x;
int32_t y;
} Foo;
```
Found:
```
// Empty file with some includes
```
The `Foo` will appear if I use it in a function signature, and will appear as extern struct (`typedef struct Foo Foo`) if I use it behind a pointer in a signature, so it might be intentional, but I would argue that cheadergen should always emit full struct definitions for `#[repr(C)] pub` structs, for public field it should use `name` and for private field it should use `__name`, and for `#[repr(Rust)]` structs it should emit an extern struct. What do you think?

No source file or test is named. Start by tracing how cheadergen handles standalone #[repr(C)] structs versus structs used in function signatures; compare the expected Foo typedef, visibility naming, and #[repr(Rust)] extern-struct behavior, then add coverage for the requested outcomes.

Written by the indexing model from the issue text.