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.

meson.build

@@ -13,6 +13,7 @@ project(
 
 inc = include_directories('include')
 nocal_sources = files(
+  'src/domain/calendar_transfer.cpp',
   'src/domain/date.cpp',
   'src/domain/event_query.cpp',
   'src/domain/month_layout.cpp',
@@ -30,6 +31,10 @@ nocal_executable = executable('nocal', 'src/main.cpp', include_directories: inc,
 domain_tests = executable('domain-tests', 'tests/domain_tests.cpp',
   include_directories: inc, link_with: nocal_lib)
 test('domain', domain_tests)
+calendar_transfer_tests = executable('calendar-transfer-tests',
+  'tests/calendar_transfer_tests.cpp', include_directories: inc,
+  link_with: nocal_lib)
+test('calendar-transfer', calendar_transfer_tests)
 ics_tests = executable('ics-tests', 'tests/ics_tests.cpp',
   include_directories: inc, link_with: nocal_lib)
 test('ics-storage', ics_tests)
@@ -46,6 +51,8 @@ test('editor', editor_tests)
 shell = find_program('sh')
 test('cli-recovery', shell,
   args: [files('tests/cli_recovery_test.sh'), nocal_executable])
+test('cli-transfer', shell,
+  args: [files('tests/cli_transfer_test.sh'), nocal_executable])
 
 install_data('scripts/nocal-hyprland', install_dir: get_option('bindir'),
   install_mode: 'rwxr-xr-x')