Add daily health summaries aggregated from HealthRecord #32

Part of #6.

Slice: aggregate the top-level HealthRecord values parsed in #30 into one summary per calendar day, so the export pipeline produces usable daily totals instead of a flat record list.

Scope:

• A Foundation-only DailyHealthSummary value type in the core package (Codable, Equatable, Sendable).
• Daily totals for steps, active and basal energy (kcal), and distance (metres), using the normalised values already on HealthRecord.
• Daily heart rate average from samples, resting heart rate (latest sample of the day), body mass (latest sample of the day), and total sleep duration from sleep analysis records.
• A HealthExport.dailySummaries(calendar:) entry point that groups records by start-of-day and returns summaries sorted by day.
• Unit tests covering grouping, totals, averages, latest-value picks, empty input, and records without dates.

Left out on purpose: CSV or JSON file writing, and feeding summaries into recommendations. Those stay on #6 for later slices.