feat(presenter): VAAPI dmabuf → Vulkan zero-copy import + CSC pass (phase 2)

The decoder's NV12 dmabuf imports per-plane (R8 + GR88, explicit DRM
format modifier, dedicated dup'd-fd import, FOREIGN→graphics acquire)
and a fullscreen-triangle render pass converts it into the presenter's
video image with the CICP-driven coefficients ported from video_gl.rs
(same tests, plus a rows-vs-matrix agreement check). SPIR-V is committed
(shaders/build.sh regenerates) so builds and CI need no toolchain. The
import extension set is probed at device creation; unsupported boxes and
3-failure streaks demote the decoder to software via the existing
force_software contract. The session binary now honors the Settings
decoder preference instead of forcing software.

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

clients/linux-session/src/main.rs

@@ -180,10 +180,10 @@ mod session_main {
                 audio_channels: settings.audio_channels,
                 preferred_codec: settings.preferred_codec(),
                 mic_enabled: settings.mic_enabled,
-                // Phase 1 presents the software path only (the Vulkan dmabuf import is
-                // Phase 2) — request it outright instead of demoting after one frame.
+                // The Settings preference (auto → VAAPI where it exists; the presenter
+                // demotes to software on boxes whose Vulkan can't import the dmabufs).
                 // PUNKTFUNK_DECODER still overrides inside the decoder for bisects.
-                decoder: "software".into(),
+                decoder: settings.decoder.clone(),
                 launch,
                 pin: Some(pin),
                 identity,