windows-metadata winrt metadata roundtrip validation

[kennykerr]

Makes RDL the reviewable source of truth for all three metadata families (WinRT, Win32, WDK), with git diff enforcing lossless RDL ↔ winmd round-trips in CI.

What changed

• tool_windows → tool_winrt: WinRT's generator now inserts an RDL step so it's structurally identical to tool_win32 / tool_wdk:

  SDK Contracts winmds ──merge──▶ merged winmd (target/)
                       ──writer, split by namespace──▶ metadata/winrt/*.rdl   (committed)
                       ──reader──▶ crates/libs/bindgen/default/Windows.winmd  (committed, derived)
  

• New metadata/winrt/ (343 per-namespace .rdl files): the committed, human-reviewable WinRT source of truth, mirroring metadata/win32 and metadata/wdk.
• New tool_roundtrip: a fast, SDK-free validator (depends only on windows-rdl — no libclang, no NuGet) that re-derives every committed RDL corpus from its committed winmd and relies on git diff.
• Removed tool_rdl_roundtrip: bitrotted, superseded by tool_roundtrip.
• CI: added winrt and roundtrip to the .github/workflows/gen.yml matrix.
• Docs: docs/crates/windows-rdl.md (design + study) and crates/libs/bindgen/default/readme.md.

One-time regen (expected)

• Windows.winmd re-bases to its RDL-rebuilt form (semantically identical — decompiling old vs new winmd yields byte-identical RDL).
• The RDL accessor shorthand doesn't record accessor parameter names, so the reader synthesizes canonical names. This normalizes generated params in the windows crate (value for property setters, handler/token for event add/remove) — cosmetic only: ABI, signatures, types, and vtable order are unchanged. windows-sys is untouched.
• Stable thereafter: re-running tool_winrt → tool_package produces zero drift.

Validation

• tool_winrt / tool_package idempotent (no drift on re-run).
• tool_roundtrip: WinRT/Win32/WDK reproduce with clean git diff.
• cargo clippy -p tool_winrt -p tool_roundtrip --all-targets clean; fmt clean.