Added documentation file explaining why period calculation functions
are tested via integration tests rather than unit tests.
Rationale:
- Period building requires full coordinator context (TimeService, price_context)
- Complex enriched price data with multiple calculated fields
- Helper functions (split_intervals_by_day, calculate_reference_prices)
are simple transformations that can't fail independently
- Integration tests provide better coverage than mocked unit tests
Testing strategy:
- test_midnight_periods.py: Period calculation across day boundaries
- test_midnight_turnover.py: Cache invalidation and recalculation
- docs/development/period-calculation-theory.md: Algorithm documentation
Impact: Clarifies testing approach for future contributors. Prevents
wasted effort on low-value unit tests for complex integrated functions.
