fix(presenter): order the vk-frame layout transition after the decode semaphore

The submit waits the Vulkan-Video frame's decode-complete timeline semaphore
at FRAGMENT_SHADER, but the acquire barrier ran its layout transition
(VIDEO_DECODE_DST/DPB -> SHADER_READ_ONLY) from srcStage TOP_OF_PIPE — which
the wait does not cover, so no dependency chain formed and the transition
could execute while the decode queue was still writing the image. The race
window is real: the pump ships each frame to the presenter BEFORE its
decode-complete wait (that wait is stats-only).

On RADV the transition physically touches the image (metadata /
decompression), so the race rendered green/yellow block corruption exactly
at freshly-decoded regions — the Steam Deck 'artifacts around the moving
cursor' bug (the cursor is the only damage on an idle desktop). NVIDIA
treats the transition as a no-op, which is why the identical code looked
clean on the dev box.

srcStage is now FRAGMENT_SHADER, matching the semaphore's wait mask — the
standard acquire pattern (same as swapchain acquires) that chains the
transition after the decode signal.

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

crates/pf-presenter/src/vk.rs

@@ -1795,6 +1795,17 @@ fn unlock_vkframe(f: &VkVideoFrame, sync: &VkFrameSync, submitted: bool, graphic
 /// sharing) and transition it for sampling. `src_qf == dst_qf` (or IGNORED/CONCURRENT)
 /// degrades to a plain layout transition. The matching decode-side acquire happens in
 /// FFmpeg, keyed off the queue_family we write back after submission.
+///
+/// `srcStage` is FRAGMENT_SHADER — NOT TOP_OF_PIPE — deliberately: the submit waits the
+/// frame's decode-complete timeline semaphore with `wait_dst_stage_mask =
+/// FRAGMENT_SHADER`, and a semaphore wait only orders operations whose first sync scope
+/// INTERSECTS that mask (the dependency-chain rule). With TOP_OF_PIPE the barrier's
+/// layout transition (VIDEO_DECODE_DST/DPB → SHADER_READ_ONLY) formed no chain with the
+/// wait and could execute while the decode queue was still writing the image. On RADV
+/// that transition physically touches the image (metadata/decompression), so the race
+/// showed as green/yellow block corruption exactly at freshly-decoded (damaged) regions
+/// — the Steam Deck cursor-trail artifact. NVIDIA treats the transition as a no-op,
+/// which is why the same code looked clean there.
 fn vkframe_acquire_barrier(
     device: &ash::Device,
     cmd: vk::CommandBuffer,
@@ -1820,7 +1831,7 @@ fn vkframe_acquire_barrier(
     unsafe {
         device.cmd_pipeline_barrier(
             cmd,
-            vk::PipelineStageFlags::TOP_OF_PIPE,
+            vk::PipelineStageFlags::FRAGMENT_SHADER,
             vk::PipelineStageFlags::FRAGMENT_SHADER,
             vk::DependencyFlags::empty(),
             &[],