docs(windows): secure-desktop two-process design + WGC impersonation attempt (vestigial)

Validated design for adding secure-desktop (UAC/lock/login) coverage on top of the shipped WGC
animation fix. Key verified constraint: WGC won't activate under SYSTEM (0x80070424) even with
thread-level ImpersonateLoggedOnUser, and DDA+SendInput on Winlogon need LOCAL_SYSTEM — so one
process can't do both. Architecture: SYSTEM host (QUIC + SudoVDA + DDA-secure + SendInput + AU mux)
+ a USER-session WGC helper (CreateProcessAsUser) that relays encoded Annex-B AUs over a named pipe;
the host muxes helper-AUs (normal desktop) vs its own DDA encoder (secure desktop), switched by a
desktop-name watcher. No shared GPU texture (rejected — MIC/keyed-mutex pain); just AU bytes.
docs/windows-secure-desktop.md has the ordered, box-testable steps.

The impersonate_active_user() in wgc.rs is kept as a harmless no-op (under a user-token process
WTSQueryUserToken fails → no impersonation → WGC works natively); it does NOT make WGC work under
SYSTEM (the two-process design uses a real user process for WGC instead). + Win32_System_RemoteDesktop.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

crates/punktfunk-host/Cargo.toml

@@ -134,6 +134,8 @@ windows = { version = "0.62", features = [
     "Graphics_DirectX_Direct3D11",
     "Win32_System_WinRT_Direct3D11",
     "Win32_System_WinRT_Graphics_Capture",
+    # WGC runs under SYSTEM via interactive-user impersonation (WGC won't activate as SYSTEM).
+    "Win32_System_RemoteDesktop",
 ] }
 # Software H.264 encoder (GPU-less path + NVENC fallback). The default `source` feature statically
 # compiles OpenH264 (BSD-2) — no system lib, builds on MSVC; nasm on PATH adds the SIMD fast path.