[johnnyhuy]

Adds CSV and JSON file output for daily health summaries. An Array extension on DailyHealthSummary provides csvString, jsonData, writeCSV and writeJSON, so the export pipeline from parsed records to daily summaries now ends in usable files.

Part of #6. Closes #34.

How it was verified:

• mise run lint - 503 violations, 11 serious in 128 files, same baseline as origin/main. No violations in the two added files.
• mise run build - passed, Build complete.
• mise run test - passed, 142 tests with 0 failures. The 9 new DailyHealthSummaryExportTests are listed below.
• mise run ios:build:dev and mise run ios:test:ui were not run. Nothing user-facing changed. This slice is core export logic only.

Proof, terminal capture from mise run test filtered to the new suite:

Test Case '-[BetterFitTests.DailyHealthSummaryExportTests testCSVEmptyArrayIsHeaderOnly]' passed.
Test Case '-[BetterFitTests.DailyHealthSummaryExportTests testCSVEmptyFieldsForMissingValues]' passed.
Test Case '-[BetterFitTests.DailyHealthSummaryExportTests testCSVHeaderAndRows]' passed.
Test Case '-[BetterFitTests.DailyHealthSummaryExportTests testCSVPreservesArrayOrder]' passed.
Test Case '-[BetterFitTests.DailyHealthSummaryExportTests testExportFromParsedExport]' passed.
Test Case '-[BetterFitTests.DailyHealthSummaryExportTests testJSONEmptyArray]' passed.
Test Case '-[BetterFitTests.DailyHealthSummaryExportTests testJSONRoundTrip]' passed.
Test Case '-[BetterFitTests.DailyHealthSummaryExportTests testJSONUsesISO8601DatesAndSortedKeys]' passed.
Test Case '-[BetterFitTests.DailyHealthSummaryExportTests testWriteCSVAndJSONToDisk]' passed.
Executed 142 tests, with 0 failures (0 unexpected)

Each test name states what it shows. Together they cover the CSV header and row values, empty fields for nil values instead of zeros, a header-only file for empty input, preserving array order, ISO8601 dates with sorted keys in JSON, a JSON round-trip back to the same summaries, writing both formats to disk and reading them back, and an end-to-end run from HealthExportParser through dailySummaries into both exports.

Reviewer notes:

• Deliberately left out: feeding summaries into recommendations, and any UI for picking or sharing a file. Those remain on Build Apple Health export ingestion and analysis pipeline #6 for later slices.
• CSV dates are ISO8601 and missing values are empty fields. Integral numbers print without a trailing .0 so step and second counts stay readable in a spreadsheet.
• Parent epic Build Apple Health export ingestion and analysis pipeline #6 stays open. This closes only the child slice Add CSV and JSON export for daily health summaries #34.
• Nothing user-facing changed, so no screenshot evidence applies. Capture method (c) unit test output succeeded. The simulator path was not attempted for this core-only change.