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/error.h

@@ -0,0 +1,36 @@
+#pragma once
+
+#include <string>
+#include <variant>
+
+namespace sc {
+
+struct CaptureError {
+    std::string message;
+};
+
+// C++20 does not provide std::expected. Use a variant-based result type so
+// fallible capture operations do not rely on exceptions.
+template <typename T> using CaptureResult = std::variant<T, CaptureError>;
+
+template <typename T> constexpr bool is_capture_error(const CaptureResult<T>& result) noexcept {
+    return std::holds_alternative<CaptureError>(result);
+}
+
+template <typename T> T& capture_value(CaptureResult<T>& result) {
+    return std::get<T>(result);
+}
+
+template <typename T> const T& capture_value(const CaptureResult<T>& result) {
+    return std::get<T>(result);
+}
+
+template <typename T> CaptureError& capture_error(CaptureResult<T>& result) {
+    return std::get<CaptureError>(result);
+}
+
+template <typename T> const CaptureError& capture_error(const CaptureResult<T>& result) {
+    return std::get<CaptureError>(result);
+}
+
+} // namespace sc