Add SAL annotation extraction to windows-rdl clang parser

[Copilot]

Win32 parameters are annotated with SAL macros (_In_, _Out_, _Inout_, _In_opt_, etc.) that encode direction and optional metadata. The windows-rdl clang parser previously ignored these, losing parameter direction information when generating RDL from C/C++ headers.

Core changes

• clang/sal.rs (new): ParamAnnotation { in_param, out_param, optional } struct + extract_sal(cursor, tu) that checks CXCursor_AnnotateAttr children (from __attribute__((annotate("_In_"))) portable stubs) and CXCursor_UnexposedAttr children (from MSVC __declspec(...) SAL expansion). Maps ~40 common SAL macros to flags.

• clang/field.rs: Param promoted from a type alias for Field to a proper struct with an annotation: ParamAnnotation field.

• clang/fn.rs: parse() calls extract_sal() per CXCursor_ParmDecl; write() emits #[in]/#[out]/#[opt] using the same logic as writer::write_params() to guarantee stable roundtrips. New sal_attrs_for_param() helper shared with interface.rs.

• clang/interface.rs, clang/callback.rs: updated to construct Param with annotation field.

Example

Given a header with SAL stubs:

#define _In_     __attribute__((annotate("_In_")))
#define _Inout_  __attribute__((annotate("_Inout_")))
#define _In_opt_ __attribute__((annotate("_In_opt_")))

void ReadBuffer(_In_ int* data, int count);
void Transform(_Inout_ int* value);
void LookupByName(_In_opt_ const char* name);

The clang parser now emits:

extern fn ReadBuffer(#[in] data: *mut i32, count: i32);
extern fn Transform(#[in] #[out] value: *mut i32);
extern fn LookupByName(#[opt] name: *const i8);

Test + docs

• New roundtrip test sal.h/sal.rdl exercising _In_, _Out_, _Inout_, _In_opt_, _Out_opt_.
• rdl.md: new "Parameter Direction Attributes" section documenting #[in], #[out], #[opt] and their SAL correspondence.