feat(capture): add Phase 3 PipeWire capture stub and config string safety

Add a linkable CaptureFactory stub so the capture API can be consumed

without unresolved symbols. Change remaining config string_view fields to

std::string to prevent dangling references. Update handoff memory.

include/screencast/capture/capture.h

@@ -3,26 +3,31 @@
 #include <cstdint>
 #include <memory>
 #include <optional>
-#include <span>
-#include <string_view>
+#include <string>
 #include <variant>
 #include <vector>
 
 namespace sc {
 
+enum class PixelFormat {
+    Rgba,
+    Yuv420p,
+};
+
 // Opaque resource owned by the capture implementation.
 struct CapturedFrame {
     int width = 0;
     int height = 0;
     uint64_t timestamp_ns = 0; // capture clock, monotonic
+    PixelFormat pixel_format = PixelFormat::Rgba;
+    int stride = 0; // bytes per row for the first plane; 0 means packed
     std::vector<std::byte> pixels;
-    // TODO: pixel format, stride, FD handle for dmabuf
 };
 
 // Capture target: whole monitor, specific window, or a region.
 struct CaptureTargetWholeScreen {};
 struct CaptureTargetWindow {
-    std::string_view window_id;
+    std::string window_id;
 };
 struct CaptureTargetRegion {
     int x = 0;
@@ -31,13 +36,10 @@ struct CaptureTargetRegion {
     int height = 0;
 };
 
-using CaptureTarget = std::variant<
-    CaptureTargetWholeScreen,
-    CaptureTargetWindow,
-    CaptureTargetRegion>;
+using CaptureTarget = std::variant<CaptureTargetWholeScreen, CaptureTargetWindow, CaptureTargetRegion>;
 
 class CaptureSession {
-public:
+  public:
     virtual ~CaptureSession() = default;
 
     // Blocking call to acquire one frame. Returns std::nullopt on graceful stop.
@@ -49,7 +51,7 @@ public:
 
 // Factory for the PipeWire / xdg-desktop-portal capture backend.
 class CaptureFactory {
-public:
+  public:
     static std::unique_ptr<CaptureSession> create(CaptureTarget target);
 };