Add #[overload("Name")] pseudo-attribute for WinRT method overloads in RDL

[Copilot]

WinRT's OverloadAttribute identifies the logical shared name for a group of uniquely-named vtable methods. RDL previously emitted this as a verbose qualified form (#[Windows::Foundation::Metadata::Overload("Method")]), which is awkward and requires Windows.Foundation.Metadata to be loaded as a reference to round-trip.

This introduces a first-class #[overload("Name")] pseudo-attribute—following the same pattern as #[typedef], #[special], and #[no_guid]—that cleanly expresses the overload grouping using the actual WINMD method names:

#[winrt]
mod Contoso {
    interface ISprocketFactory {
        #[overload("Create")]
        fn CreateDefault(&self) -> Sprocket;
        #[overload("Create")]
        fn CreateWithOptions(&self, options: SprocketOptions) -> Sprocket;
    }
}

In RDL the fn identifier is the unique vtable name and #[overload("...")] carries the common logical name. This matches the WinRT metadata binary layout where MethodDef.Name stores the common name and OverloadAttribute.value stores the unique name.

Changes:

• Writer (crates/libs/rdl/src/writer/interface.rs): write_method detects OverloadAttribute, uses its value as the fn name (unique vtable name), and emits #[overload("common")] from MethodDef.Name. OverloadAttribute is excluded from the general custom-attributes path.
• Reader (crates/libs/rdl/src/reader/interface.rs): encode_interface detects #[overload("common")], uses the arg as MethodDef.Name, and emits OverloadAttribute with the fn identifier (unique name) as its value. GUID derivation uses MethodDef.Name for consistency.
• Roundtrip test (crates/tests/libs/rdl/tests/roundtrip/overload-method.rdl): Golden exercising the full RDL → WINMD → RDL cycle.
• Encoding test (crates/tests/libs/rdl/tests/overload_encoding.rs): Directly reads the produced WINMD and asserts that MethodDef.Name = common name and OverloadAttribute.value = unique name.
• Docs (crates/libs/rdl/rdl.md): Documents #[overload] under the Interfaces section.

[kennykerr]

@copilot this .rdl looks right but the .winmd representation is such that the MethodDef name stores the common name while the attribute stores the unique name. So keep this .rdl representation but make sure the metadata is actually encoded correctly.

[Copilot]

Fixed in 912ca1a. The WINMD encoding is now correct:

• MethodDef.Name = common/logical name (e.g. "Method") — taken from the #[overload("...")] arg
• OverloadAttribute.value = unique vtable name (e.g. "MethodOne") — taken from the fn identifier

Added overload_encoding.rs which directly reads the produced WINMD and asserts this layout.

[kennykerr]

@copilot fmt code

[Copilot]

@copilot fmt code

Done in 870a4da.