refactor(host/windows): collapse Windows capture to IDD-push only

Remove DXGI Desktop Duplication (DuplCapturer), Windows.Graphics.Capture
(WgcCapturer), the two-process SYSTEM+helper relay (virtual_stream_relay /
HelperRelay / DesktopWatcher / composed_flip), and the five source files that
implemented them. IDD direct-push is now the sole Windows capture path; the
session topology is always SingleProcess.

Deleted files: wgc.rs, wgc_relay.rs, desktop_watch.rs, composed_flip.rs,
windows/wgc_helper.rs (+ wgc-helper subcommand in main.rs).

dxgi.rs is kept but carved to shared GPU primitives only (make_device,
HdrP010Converter, VideoConverter, install_gpu_pref_hook, WinCaptureTarget,
pack_luid) — ~2237 lines of DDA-only code removed; imports cleaned.

capture.rs: IDD-push open failure fails the session cleanly (no fallback).
Adds capturer_supports_444() — returns false on Windows (IDD-push 4:4:4 is a
follow-up), replacing the stale single_process gate in 4:4:4 negotiation.
session_plan.rs: CaptureBackend{Dda,Wgc} and SessionTopology::TwoProcessRelay
removed. config.rs: no_helper/force_helper/no_wgc/capture_backend/secure_dda
removed. merged_env_block relocated from wgc_relay to windows/interactive.rs.

Linux cargo check clean.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

crates/punktfunk-host/src/main.rs

@@ -56,9 +56,6 @@ mod spike;
 mod stats_recorder;
 mod vdisplay;
 #[cfg(target_os = "windows")]
-#[path = "windows/wgc_helper.rs"]
-mod wgc_helper;
-#[cfg(target_os = "windows")]
 #[path = "windows/win_adapter.rs"]
 mod win_adapter;
 #[cfg(target_os = "windows")]
@@ -392,35 +389,6 @@ fn real_main() -> Result<()> {
                 paired_store: None,
             })
         }
-        // USER-session WGC helper (Windows two-process secure-desktop design): capture the EXISTING
-        // SudoVDA via WGC + NVENC, stream AUs on stdout to the SYSTEM host. Spawned by the host
-        // (CreateProcessAsUser), not run by hand. See design/archive/windows-secure-desktop.md.
-        #[cfg(target_os = "windows")]
-        Some("wgc-helper") => {
-            let get = |flag: &str| {
-                args.iter()
-                    .skip_while(|a| *a != flag)
-                    .nth(1)
-                    .map(String::as_str)
-            };
-            let (width, height, fps) = get("--mode")
-                .and_then(|m| {
-                    let p: Vec<u32> = m.split('x').filter_map(|s| s.parse().ok()).collect();
-                    (p.len() == 3).then(|| (p[0], p[1], p[2]))
-                })
-                .unwrap_or((1920, 1080, 60));
-            wgc_helper::run(wgc_helper::HelperOptions {
-                target_id: get("--target-id").and_then(|s| s.parse().ok()).unwrap_or(0),
-                gdi_name: get("--gdi").unwrap_or("").to_string(),
-                width,
-                height,
-                fps,
-                bitrate_kbps: get("--bitrate")
-                    .and_then(|s| s.parse().ok())
-                    .unwrap_or(20000),
-                bit_depth: get("--bit-depth").and_then(|s| s.parse().ok()).unwrap_or(8),
-            })
-        }
         // Windows service control: install/uninstall/start/stop/status + the SCM `run` entry point.
         // Replaces the ad-hoc launch chain — `service install` registers an auto-start SYSTEM service
         // that launches the host into the active interactive session.