Count only asleep time in daily Health export summaries #38

Part of #6.

DailyHealthSummary currently sums every sleep-analysis interval, so in-bed and awake records inflate sleep duration and overlapping stage or device records are counted twice.

Complete one core-only slice:

• Count recognised asleep categories, including legacy asleep and current sleep stages.
• Exclude awake, in-bed, unknown categories and invalid or incomplete intervals.
• Merge overlapping asleep intervals within each existing start-date calendar-day bucket.
• Cover the XML parser to daily summary to CSV/JSON path with tests, preserving nil when no valid asleep interval is present.

Keep the existing start-date day allocation. Cross-midnight splitting, source prioritisation and UI are outside this slice.