vk_native: window-space view-cache eviction can free in-flight descriptor sets on the split path (+ #1783 nits) #1792

Follow-up from the #1783 review.

1. The split path can free descriptor sets that in-flight frames still use. vk_compositor_forget_dead_window_space_images (#1783) assumes the previous frame's submit has always been waited on before the window-space pass records. That's true for the windowed path (fence), shared-texture (vkQueueWaitIdle) and DP self-submit. It's not true for vk_split_composite_window_space: its VK_SPLIT_PLANE_CMD_RING (3 deep) only waits on the slot it reuses, so up to two earlier command buffers may still be executing.

Failure: on a hybrid box with the weave-on-scanout split engaged, more than 64 window-space images get destroyed between two passes. The overflow branch then frees the descriptor sets of every cached entry, including live HUD images bound by an in-flight command buffer (VUID-vkFreeDescriptorSets-pDescriptorSets-00309). The neighbouring atlas-framebuffer eviction comment makes the same "always waited" claim.

Fix: on the split path, wait for the outstanding ws_cmd_value[] before forgetting, or route eviction through a deferred-free list keyed on timeline values. At minimum, correct both comments.

2. Nits from the same review:

• The overflow loop while (image_count > 0) forget(cached_images[0].image) spins forever if entry 0's image is VK_NULL_HANDLE (unreachable today). Add a vk_hud_blend_forget_all().
• The doxygen block for vk_compositor_render_window_space_into_atlas is now separated from its function by the new function.

3. Pre-existing:

• vk_swapchain_destroy calls vkDestroyImage with no GPU wait.
• comp_multi_system's hud_blend / chrome_blend / shared_chrome_blend caches never evict.