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/settings.rs

@@ -19,11 +19,18 @@ const RESOLUTIONS: &[(u32, u32)] = &[
 /// `0` = the display's native refresh, resolved at connect.
 const REFRESH: &[u32] = &[0, 30, 60, 90, 120, 144, 165, 240];
 /// Decode backend presets: `(stored value, display label)`.
+// A stored legacy "hardware" (the D3D11VA era) matches no preset, so the combo shows
+// Automatic — which is exactly how the session's decoder chain reads that value.
 const DECODERS: &[(&str, &str)] = &[
     ("auto", "Automatic (GPU, fall back to CPU)"),
-    ("hardware", "Hardware (GPU / D3D11VA)"),
+    ("vulkan", "Hardware (GPU / Vulkan Video)"),
     ("software", "Software (CPU)"),
 ];
+/// Temporary A/B knob (see `Settings::engine`) — deleted with the legacy path.
+const ENGINES: &[(&str, &str)] = &[
+    ("", "Vulkan session window (recommended)"),
+    ("builtin", "Built-in D3D11VA (legacy)"),
+];
 /// Audio channel presets: `(channel count, display label)`. The host clamps to what it can
 /// capture; the resolved count drives the decoder + WASAPI render layout.
 const AUDIO_CHANNELS: &[(u8, &str)] = &[(2, "Stereo"), (6, "5.1 Surround"), (8, "7.1 Surround")];
@@ -182,8 +189,8 @@ pub(crate) fn settings_page(
         s.decoder = DECODERS[i].0.to_string();
     })
     .tooltip(
-        "Hardware decode (D3D11VA) is far lighter than software \u{2014} keep it on Automatic \
-         unless debugging.",
+        "Hardware decode (Vulkan Video) is far lighter than software \u{2014} keep it on \
+         Automatic unless debugging.",
     );
     // GPU picker, only on a multi-GPU box (hybrid laptop, eGPU): which adapter decodes + presents.
     // Stored as the adapter description; empty = automatic (the window's monitor's adapter).
@@ -239,6 +246,15 @@ pub(crate) fn settings_page(
         "Advertise 10-bit HDR10 so the host upgrades HDR content. Needs a display in HDR mode; \
          SDR content is unaffected.",
     );
+    let (eng_names, eng_i) = presets(ENGINES, |v| *v == s.engine);
+    let engine_combo = setting_combo(ctx, "Streaming engine", eng_names, eng_i, |s, i| {
+        s.engine = ENGINES[i].0.to_string();
+    })
+    .tooltip(
+        "Temporary: compare the Vulkan session window against the legacy in-process \
+         D3D11VA presenter. Applies to the next stream. This option goes away once the \
+         Vulkan path is fully validated.",
+    );
 
     // --- Input -----------------------------------------------------------------------------
     // Which physical controller forwards as pad 0: automatic = the most recently connected;
@@ -340,6 +356,7 @@ pub(crate) fn settings_page(
                     bitrate_box.into(),
                     hdr_toggle.into(),
                     hud_toggle.into(),
+                    engine_combo.into(),
                 ]);
                 controls
             }),