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.

src/capture/pipewire_capture.cpp

@@ -4,10 +4,10 @@
 
 namespace sc {
 
-std::unique_ptr<CaptureSession> CaptureFactory::create(CaptureTarget /*target*/) {
+CaptureResult<std::unique_ptr<CaptureSession>> CaptureFactory::create(CaptureTarget /*target*/) {
     // Phase 3 placeholder. The PipeWire / xdg-desktop-portal capture backend
-    // will be implemented here once the codec round-trip is solidified.
-    return nullptr;
+    // will be implemented here once the portal integration lands.
+    return CaptureError{"PipeWire capture is not implemented yet"};
 }
 
 } // namespace sc