feat(vulkan): select readback-capable swapchains

adapters/fparkan-render-vulkan/src/ffi/smoke.rs

@@ -241,6 +241,7 @@ impl VulkanSmokeRenderer {
                 enabled_extension_count: 0,
                 swapchain_extent: (0, 0),
                 swapchain_image_count: 0,
+                swapchain_image_usage: 0,
             },
         };
         renderer.rebuild_swapchain_resources(false)?;
@@ -268,6 +269,7 @@ impl VulkanSmokeRenderer {
                 .unwrap_or(u32::MAX),
             swapchain_extent: swapchain_ref.report.plan.extent,
             swapchain_image_count: swapchain_ref.report.image_count,
+            swapchain_image_usage: swapchain_ref.report.plan.image_usage,
         };
         Ok(renderer)
     }
@@ -571,11 +573,13 @@ impl VulkanSmokeRenderer {
         let frame_sync = create_frame_sync(device)?;
         let swapchain_extent = self.swapchain_ref()?.report.plan.extent;
         let swapchain_image_count = self.swapchain_ref()?.report.image_count;
+        let swapchain_image_usage = self.swapchain_ref()?.report.plan.image_usage;
         let resources = resources.commit();
         self.images_in_flight = vec![vk::Fence::null(); resources.image_views.len()];
         self.frame_sync = frame_sync;
         self.report.swapchain_extent = swapchain_extent;
         self.report.swapchain_image_count = swapchain_image_count;
+        self.report.swapchain_image_usage = swapchain_image_usage;
         self.swapchain_resources = Some(resources);
         Ok(())
     }

adapters/fparkan-render-vulkan/src/ffi/smoke_types.rs

@@ -457,6 +457,8 @@ pub struct VulkanSmokeRendererReport {
     pub swapchain_extent: (u32, u32),
     /// Current swapchain image count.
     pub swapchain_image_count: u32,
+    /// Current swapchain image-usage flags as raw Vulkan bits.
+    pub swapchain_image_usage: u32,
 }
 
 /// Measured validation counters from the live smoke loop.

adapters/fparkan-render-vulkan/src/ffi/swapchain.rs

@@ -185,7 +185,7 @@ pub fn create_vulkan_swapchain_probe_for_extent(
             height: plan.extent.1,
         })
         .image_array_layers(1)
-        .image_usage(vk::ImageUsageFlags::COLOR_ATTACHMENT)
+        .image_usage(vk::ImageUsageFlags::from_raw(plan.image_usage))
         .image_sharing_mode(sharing_mode)
         .queue_family_indices(&queue_family_indices)
         .pre_transform(raw_capabilities.current_transform)

adapters/fparkan-render-vulkan/src/ffi/tests.rs

@@ -94,6 +94,7 @@ fn frame_submission_plan_json_is_stable() -> Result<(), RenderError> {
         },
         present_mode: vk::PresentModeKHR::FIFO.as_raw(),
         image_count: 3,
+        image_usage: vk::ImageUsageFlags::COLOR_ATTACHMENT.as_raw(),
     };
 
     let plan = plan_vulkan_frame_submission(&swapchain, &commands)?;
@@ -543,6 +544,23 @@ fn swapchain_plan_prefers_srgb_mailbox_and_clamps_extent() {
     assert_eq!(plan.present_mode, vk::PresentModeKHR::MAILBOX.as_raw());
     assert_eq!(plan.extent, (1024, 720));
     assert_eq!(plan.image_count, 3);
+    assert_eq!(
+        plan.image_usage,
+        vk::ImageUsageFlags::COLOR_ATTACHMENT.as_raw()
+    );
+}
+
+#[test]
+fn swapchain_plan_enables_transfer_source_when_surface_supports_it() {
+    let mut request = swapchain_request();
+    request.capabilities.supported_usage_flags |= vk::ImageUsageFlags::TRANSFER_SRC.as_raw();
+
+    let plan = plan_vulkan_swapchain(&request).expect("swapchain plan");
+
+    assert_eq!(
+        plan.image_usage,
+        (vk::ImageUsageFlags::COLOR_ATTACHMENT | vk::ImageUsageFlags::TRANSFER_SRC).as_raw()
+    );
 }
 
 #[test]
@@ -606,7 +624,7 @@ fn swapchain_plan_json_and_recreation_reports_are_stable() {
     let plan = plan_vulkan_swapchain(&swapchain_request()).expect("swapchain plan");
     assert_eq!(
         render_swapchain_plan_json(&plan),
-        "{\"schema\":1,\"extent\":[1024,720],\"format\":50,\"color_space\":0,\"present_mode\":1,\"image_count\":3}"
+        "{\"schema\":1,\"extent\":[1024,720],\"format\":50,\"color_space\":0,\"present_mode\":1,\"image_count\":3,\"image_usage\":16}"
     );
 
     let report = swapchain_recreation_report(

adapters/fparkan-render-vulkan/src/planning_backend.rs

@@ -163,5 +163,6 @@ fn default_stage0_swapchain_plan() -> VulkanSwapchainPlan {
         },
         present_mode: vk::PresentModeKHR::FIFO.as_raw(),
         image_count: 2,
+        image_usage: vk::ImageUsageFlags::COLOR_ATTACHMENT.as_raw(),
     }
 }

adapters/fparkan-render-vulkan/src/policy.rs

@@ -96,6 +96,8 @@ pub struct VulkanSwapchainPlan {
     pub present_mode: i32,
     /// Selected image count.
     pub image_count: u32,
+    /// Selected image-usage flags as raw Vulkan bits.
+    pub image_usage: u32,
 }
 
 /// Swapchain planning error.
@@ -425,6 +427,7 @@ pub fn plan_vulkan_swapchain(
         format,
         present_mode,
         image_count: select_image_count(request.capabilities),
+        image_usage: select_swapchain_image_usage(request.capabilities),
     })
 }
 
@@ -521,6 +524,7 @@ pub fn render_swapchain_plan_json(plan: &VulkanSwapchainPlan) -> String {
         color_space: i32,
         present_mode: i32,
         image_count: u32,
+        image_usage: u32,
     }
 
     serialize_json_or_fallback(
@@ -531,8 +535,9 @@ pub fn render_swapchain_plan_json(plan: &VulkanSwapchainPlan) -> String {
             color_space: plan.format.color_space,
             present_mode: plan.present_mode,
             image_count: plan.image_count,
+            image_usage: plan.image_usage,
         },
-        "{\"schema\":0,\"extent\":[0,0],\"format\":0,\"color_space\":0,\"present_mode\":0,\"image_count\":0}",
+        "{\"schema\":0,\"extent\":[0,0],\"format\":0,\"color_space\":0,\"present_mode\":0,\"image_count\":0,\"image_usage\":0}",
     )
 }
 
@@ -809,6 +814,15 @@ fn supports_depth_stencil_request(
     select_depth_stencil_attachment_format(&device.supported_depth_stencil_formats, depth).is_some()
 }
 
+fn select_swapchain_image_usage(capabilities: VulkanSwapchainSurfaceCapabilities) -> u32 {
+    let mut usage = vk::ImageUsageFlags::COLOR_ATTACHMENT;
+    let supported = vk::ImageUsageFlags::from_raw(capabilities.supported_usage_flags);
+    if supported.contains(vk::ImageUsageFlags::TRANSFER_SRC) {
+        usage |= vk::ImageUsageFlags::TRANSFER_SRC;
+    }
+    usage.as_raw()
+}
+
 /// Selects the first canonical depth/stencil attachment format supported by a device.
 ///
 /// The order is part of the Windows Vulkan compatibility contract and is shared