Add WinRT regression test harness (portable golden net)

[Gordon Lam (yeelam-gordon)]

Summary

Adds a portable WinRT regression test — one self‑contained integration test (crates/dynwinrt/tests/winrt_regression.rs) that uses only dynwinrt's public API and asserts ~43 golden facts about real WinRT behavior. It locks current behavior so future engine/codegen changes can't silently regress it. Independent of the Win32/COM work.

Motivation

dynwinrt is a dynamic FFI + codegen engine: it marshals arbitrary WinRT calls through libffi driven by .winmd metadata. Regressions there are easy to introduce and hard to notice by eye. This net runs in CI (cargo test -p dynwinrt) and is deliberately public‑API‑only, so it stays stable across internal refactors while still exercising the full activate → dynamic‑invoke → marshal‑back path against real Windows APIs.

What it covers

• Windows.Foundation.Uri — dynamically activate and read Host / Domain / Path / SchemeName / AbsoluteUri for fixed inputs (incl. query/fragment, empty‑path).
• PropertyValue — scalar create/read round‑trips and type‑mismatch behavior.
• BasicGeoposition / Geopoint — value‑type struct layout + a dynamic round‑trip through a factory that takes a struct.

Sample (shape of an assertion)

// crates/dynwinrt/tests/winrt_regression.rs — public API only
let uri = /* dynamically activated Windows.Foundation.Uri via MetadataTable */;
assert_eq!(host, "example.com");
assert_eq!(path, "/a/b");
assert_eq!(scheme, "https");

Status

Green today: 1 integration test, ~43 assertions, alongside the existing unit suite. Base: main; independent of the classic‑COM and flat‑Win32 PRs (it is used to prove those don't regress WinRT behavior).