feat: add safe calendar import and export

Add explicit noninteractive CLI transfer modes with full event validation, idempotent duplicate handling, collision refusal, atomic import backups, and no-overwrite export creation.

Cover pure merge semantics and end-to-end rollback paths for unsupported content, invalid identities, conflicts, option combinations, and existing destinations.

include/nocal/domain/calendar_transfer.hpp

@@ -0,0 +1,28 @@
+#pragma once
+
+#include "nocal/domain/event.hpp"
+
+#include <cstddef>
+#include <span>
+#include <vector>
+
+namespace nocal {
+
+struct ImportMergeResult {
+    std::vector<Event> events;
+    std::size_t imported{};
+    std::size_t duplicates_skipped{};
+};
+
+// Transfer inputs must contain valid intervals and one non-empty, unique UID
+// per event. Invalid input throws std::invalid_argument before any persistence
+// boundary is crossed.
+void validate_events_for_export(std::span<const Event> events);
+
+// Preserves target order and appends new source events in source order. An
+// exactly equivalent source/target UID is skipped; a differing UID collision
+// throws std::invalid_argument and returns no partial result.
+[[nodiscard]] ImportMergeResult merge_events_for_import(
+    std::span<const Event> target, std::span<const Event> source);
+
+} // namespace nocal

include/nocal/domain/domain.hpp

@@ -1,5 +1,6 @@
 #pragma once
 
+#include "nocal/domain/calendar_transfer.hpp"
 #include "nocal/domain/date.hpp"
 #include "nocal/domain/event.hpp"
 #include "nocal/domain/event_query.hpp"