feat(clients/windows): port the Vulkan session client to Windows — session-always

The punktfunk-session Vulkan client (clients/linux-session, now clients/session)
builds and runs on Windows; the WinUI shell spawns it for every stream. Verified
live: 10-bit HEVC via Vulkan Video on both AMD (iGPU) and NVIDIA, 5120x1440 at
130 fps / 8 ms end-to-end on the RTX 4090.

- pf-ffvk: Windows bindgen branch (FFMPEG_DIR + PF_FFVK_VULKAN_INCLUDE, no
  pkg-config); provisioning fetches Vulkan-Headers (pinned v1.4.309).
- pf-client-core: builds on Windows — WASAPI audio (audio_wasapi.rs, cfg-swapped
  via #[path], same surface as the PipeWire twin), VAAPI/dmabuf gated inline
  (chain = vulkan -> software), trust reads the WinUI shell's %APPDATA% stores
  (parity tests pin both serialized shapes), Settings gains adapter/hdr_enabled
  (serde-defaulted; Linux stores unaffected).
- pf-presenter: builds on Windows — dmabuf module Linux-gated; SDL keyboard grab
  while captured (Alt+Tab/Win reach the host); pick_device ranks discrete over
  integrated (device 0 was the iGPU on hybrid boxes — the silent footgun) and
  honors PUNKTFUNK_VK_ADAPTER (the Settings GPU pick, exported by the session).
- run loop: block in one SDL wait woken by input AND decoded frames (a per-
  session forwarder pushes a FrameWake user event) instead of a 1 ms poll —
  measured 111%% -> 5%% of a core (NVIDIA), 86%% -> 3.5%% (AMD), stats unchanged.
  The pump's decode-fence wait became once-per-window sampling (no per-frame
  pipeline stall; the stat now shows true backlog).
- pf-console-ui: builds on Windows (skia-safe msvc prebuilts); font lookup falls
  through fontconfig aliases to concrete DirectWrite families (Consolas/Segoe UI)
  — browse/coverflow works, verified against a live host.
- WinUI shell: session-always via new src/spawn.rs (GTK spawn.rs port —
  CREATE_NO_WINDOW, stdout contract, kill handle); the Stream screen is a status
  card (chips + stage lines from the child's stats). The legacy in-process
  D3D11VA path stays behind Settings "Streaming engine" / PUNKTFUNK_BUILTIN_
  STREAM=1 as the A/B baseline until Phase 8 deletes it. SessionParams.video_caps
  makes the HDR toggle real.
- clients/linux-session renamed to clients/session (builds for both OSes).
- CI/MSIX: both workflows build/test both bins with widened path filters; the
  MSIX ships punktfunk-session.exe. ARM64 session builds --no-default-features
  (rust-skia has no aarch64-pc-windows-msvc prebuilts; flip when it does).

A/B on this box (5120x1440 HEVC vs home-worker-5): NVIDIA Vulkan 130 fps / 8 ms
e2e / 1.6 ms decode — clearly better than the built-in path. The AMD iGPU VCN
saturates at ~52 fps where its own D3D11VA does ~70 — Adrenalin Vulkan decode is
slower on APU silicon; discrete RDNA validation gates Phase 8.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

clients/windows/src/app/mod.rs

@@ -118,6 +118,12 @@ pub(crate) struct Shared {
     /// Latest stream stats, written by the session's event loop and mirrored into reactor state
     /// by the HUD poll thread to drive the overlay.
     pub(crate) stats: Mutex<Stats>,
+    /// The live session child (spawn mode) — the status page's Disconnect and the
+    /// request-access Cancel kill it. A FRESH handle is installed per spawn.
+    pub(crate) session: Mutex<crate::spawn::SessionChild>,
+    /// Latest `stats:` line from the session child (spawn mode), already formatted;
+    /// mirrored into the HUD sample for the session status page.
+    pub(crate) stats_line: Mutex<String>,
     /// Cancel flag for the in-flight "request access" connect. A FRESH flag is installed per
     /// request: the waiting screen's Cancel button reads it back from here and sets it, and that
     /// request's event loop (which captured the same `Arc` at spawn) then tears down silently when
@@ -136,6 +142,15 @@ pub struct AppCtx {
     pub(crate) shared: Arc<Shared>,
 }
 
+/// The legacy in-process streaming path (SwapChainPanel + D3D11VA) instead of the
+/// spawned punktfunk-session window: the Settings "Streaming engine" pick, or the
+/// `PUNKTFUNK_BUILTIN_STREAM=1` env override. A temporary A/B knob — both go away with
+/// the legacy path once the Vulkan session is fully validated.
+pub(crate) fn use_builtin_stream(ctx: &AppCtx) -> bool {
+    std::env::var_os("PUNKTFUNK_BUILTIN_STREAM").is_some_and(|v| v == "1")
+        || ctx.settings.lock().unwrap().engine == "builtin"
+}
+
 pub fn run(identity: (String, String), gamepad: GamepadService) -> windows_reactor::Result<()> {
     let ctx = Arc::new(AppCtx {
         identity,
@@ -254,6 +269,7 @@ fn root(cx: &mut RenderCx, ctx: &Arc<AppCtx>) -> Element {
                         captured: crate::input::is_captured(),
                         visible: crate::input::hud_visible(),
                         present: crate::render::present_stats(),
+                        stats_line: shared.stats_line.lock().unwrap().clone(),
                     });
                 })
                 .ok();
@@ -403,6 +419,11 @@ fn root(cx: &mut RenderCx, ctx: &Arc<AppCtx>) -> Element {
         Screen::Help => help::help_page(&set_screen),
         Screen::Pair => component(pair::pair_page, svc),
         Screen::SpeedTest => component(speed::speed_page, SpeedProps { svc, state: speed }),
+        // Spawn mode (the default): the stream runs in the punktfunk-session child's own
+        // window; this screen is a status page (no hooks — inline is sound). The legacy
+        // in-process SwapChainPanel page stays behind the "Streaming engine" setting /
+        // PUNKTFUNK_BUILTIN_STREAM=1.
+        Screen::Stream if !use_builtin_stream(ctx) => stream::session_page(ctx, &hud),
         Screen::Stream => component(stream::stream_page, StreamProps { svc, hud }),
     };