test: guard domain entity decoupling

[jjoonleo]

Summary

• Clarify core scheduling glossary terms in CONTEXT.md: OnTime User, Schedule, Place, Preparation, and Preparation Step.
• Strengthen the domain entity boundary test to catch forbidden relative imports from lower layers.
• Add a regression assertion that domain entities do not reintroduce legacy persistence mapper method names.

Current origin/main already contains the data-layer mapper extraction for #525; this PR hardens that accepted boundary and records the resolved domain vocabulary without adding implementation details to the glossary.

Closes #525

Tests

• flutter pub get
• dart run build_runner build --delete-conflicting-outputs
• flutter test test/domain/entities/domain_entity_boundary_test.dart test/data/mappers/domain_persistence_mappers_test.dart test/data/data_sources/schedule_local_data_source_test.dart test/presentation/alarm/utils/preparation_step_state_mapper_test.dart
• flutter test test/domain/entities test/data/mappers/domain_persistence_mappers_test.dart test/data/data_sources/schedule_local_data_source_test.dart test/data/data_sources/preparation_local_data_source_test.dart test/data/daos/user_dao_test.dart test/presentation/alarm/utils/preparation_step_state_mapper_test.dart
• flutter analyze
• rg -n "package:on_time_front/(core|data|presentation)|(?:\.\./)+(?:core|data|presentation)|fromScheduleWithPlaceModel|toScheduleModel|toScheduleWithPlaceModel|toPreparationUserModel|\bfromModel\b|\btoModel\b|\bfrom[A-Za-z0-9_]*Model\b|\bto[A-Za-z0-9_]*Model\b|PreparationStateEnum" lib/domain/entities || true