Parse activity summaries from Apple Health exports #46

Part of #6.

Parse ActivitySummary elements alongside workouts and records in the Foundation-only Apple Health export pipeline.

• Preserve the exported calendar-day string without assigning an invented time zone.
• Capture active energy and its goal with the exported unit, exercise minutes and goal, and stand hours and goal.
• Expose normalised energy in kilocalories for supported units.
• Keep older HealthExport JSON decodable when the new collection is absent.
• Prove mixed exports, missing or invalid values, energy conversion, and Codable compatibility with unit tests.

App UI, activity ring rendering, daily-record aggregation, and recommendation policy are follow-up slices. Leave the parent epic open.