feat(capture): give CaptureFactory a proper error channel

Replace the nullable unique_ptr returned by CaptureFactory::create()
with CaptureResult<std::unique_ptr<CaptureSession>> using the new
CaptureError/CaptureResult pattern, mirroring codec/error.h. The stub
now reports 'not implemented yet' as an error instead of returning
nullptr. Update handoff memory with the review outcome and
forward-looking notes for phases 3, 5, and 7.

include/screencast/capture/capture.h

@@ -1,5 +1,7 @@
 #pragma once
 
+#include "screencast/capture/error.h"
+
 #include <cstdint>
 #include <memory>
 #include <optional>
@@ -52,7 +54,7 @@ class CaptureSession {
 // Factory for the PipeWire / xdg-desktop-portal capture backend.
 class CaptureFactory {
   public:
-    static std::unique_ptr<CaptureSession> create(CaptureTarget target);
+    static CaptureResult<std::unique_ptr<CaptureSession>> create(CaptureTarget target);
 };
 
 } // namespace sc