Export parsed activity ring summaries as CSV and JSON #52

Part of #6.

Add CSV and JSON export for the parsed HealthActivitySummary values, following the existing daily and weekly summary export APIs. Keep activity ring totals separate from record-derived daily totals.

• Export the calendar date, normalised energy and goal in kilocalories, exercise minutes and goal, and stand hours and goal as CSV.
• Preserve raw units and values in Codable JSON for a lossless round trip.
• Preserve input order, missing values and measured zeroes; escape CSV text fields.
• Provide atomic file writers and document the public API.
• Prove the XML-to-export path with unit and integration tests plus the repository lint, build and test gates.

No app UI changes. Recommendation policy and activity ring analytics remain follow-up slices.