Build Unit Assignment Service with constraint enforcement

Implement atomic primary demotion as a Supabase RPC function set_primary_assignment(p_user_id uuid, p_unit_id uuid) that in a single transaction: (1) sets is_primary = false on all existing primary assignments for that user in that organization, (2) inserts or updates the new assignment with is_primary = true. This prevents the dual-primary race condition that would occur if Dart issued two sequential UPDATE/INSERT calls. In the Dart service, call this RPC via supabase.rpc('set_primary_assignment', params: {...}). The 5-chapter limit check should query COUNT from unit_assignments WHERE user_id = userId before calling the RPC — pass the snapshot count to HierarchyStructureValidator.validateUserDepthLimit() to keep the validator pure.

Inject ActiveChapterCubit via the service constructor; call switchChapter() after a successful primary assignment write to ensure the state update is not skipped on RPC error.

Unit tests using mockito/mocktail to mock UnitAssignmentRepository, HierarchyStructureValidator, and ActiveChapterCubit. Cover: (1) assign success, (2) assign with primary demotion — assert repository receives demotion call before new assignment, (3) assign rejected at depth limit — assert no repository write occurs, (4) assign idempotency, (5) unassign primary — assert ActiveChapterCubit.switchToEmpty() called, (6) unassign non-primary — assert cubit not notified, (7) listAssignments correct ordering. Integration test against local Supabase: create two chapter assignments for a test user, mark second as primary, assert first is demoted atomically. Minimum 85% branch coverage.