docs: add Plan 02 - Data Layer & Permission Flow implementation plan
21 bite-sized tasks covering domain models, CalendarContract data layer (Cursor mappers with §8 defensive validation, ContentObserver-backed SharedFlow repository), DataStore-persisted hidden-calendar set, Hilt wiring, READ_CALENDAR permission flow (rationale + denied recovery), and a wegwerfbarer Debug screen that visually validates data is flowing. Out of scope: Month/Week/Day views (Plans 03-05), Event Detail Sheet (Plan 06), Filter/Settings (Plan 07).
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