Qualify special string typedefs (PCWSTR) to Windows.Win32.Foundation for non-Win32 metadata

[Jevan Saks (jevansaks)]

Problem

When generating C# from a non-Windows.Win32 metadata assembly (e.g. a winmd built from a UDK header, common namespace OSClient), a parameter typed _In_reads_opt_(count) LPCWSTR* / [Const] PWSTR* (projected as PCWSTR*) fails to compile:

error CS0234: The type or namespace name 'Foundation' does not exist in the namespace 'OSClient'
error CS8500: This takes the address of, gets the size of, or declares a pointer to a managed type

CsWin32 qualified the special string typedef PCWSTR under the consuming assembly's own root namespace — OSClient.Foundation.PCWSTR — which does not exist. It should reference global::Windows.Win32.Foundation.PCWSTR, where the type actually lives.

Root cause

This reproduces only in the configuration OSClient uses: a single Generator (no SuperGenerator), with the Windows.Win32 projection supplied as a referenced assembly rather than a sibling generator. In that setup there is no Windows.Win32 generator to delegate special-typedef emission to, so the special typedefs must be referenced under Windows.Win32.Foundation.

Two places built the wrong qualification for the projected PCWSTR*:

1. Generator.RequestSpecialTypeDefStruct computed the fully-qualified name from this.Namespace (the consuming assembly's common namespace → OSClient). The delegation that fixes this up only runs when a SuperGenerator is present, so in the single-generator case the name stayed OSClient.Foundation.PCWSTR. This is the element type used by the extern/interface signature, the in PCWSTR friendly overload, and the fixed block. This is the primary CS0234.

2. Generator.FriendlyOverloads.PCWSTRTypeSyntax (used only by the ReadOnlySpan<string> friendly overload generated for a counted array, [Const, NativeArrayInfo] PWSTR*) hard-coded the ArrayPool<PCWSTR> element type to the winmdroot alias, which for non-Win32 metadata points at the wrong assembly.

Fix

1. RequestSpecialTypeDefStruct now qualifies special typedefs under Windows.Win32.Foundation for non-Win32 metadata (rootNamespace = IsWin32Sdk ? this.Namespace : "Windows.Win32").
2. PCWSTRTypeSyntax is now an instance member using Win32NamespacePrefix, matching how the parameter type itself is qualified.

Both changes are no-ops for the Windows.Win32 SDK itself (byte-identical output — verified by Everything_NoFriendlyOverloads).

Tests

Adds SelfContained.winmd, a single-generator P/Invoke fixture that faithfully reproduces the OSClient configuration, and a theory SingleGeneratorQualifiesSpecialTypeDefToWin32Namespace covering both projections:

• Foo — [Const] PWSTR* with no count → in PCWSTR (exercises fix Add unit tests for code generation #1).
• Bar — [Const, NativeArrayInfo] PWSTR* → ReadOnlySpan<string> + ArrayPool<PCWSTR> (exercises fix Add unit tests for code generation #1 and Consume metadata as dll #2).

Both fail with CS0234 on the pre-fix generator and compile cleanly with the fix.

Fixes the CsWin32 sanity-check failure in OSClient PR 16236433.