feat(health): add daily health summaries aggregated from records - #33

Adds daily aggregation for Apple Health exports. A new Foundation-only DailyHealthSummary value type holds one day of steps, active and basal energy, distance, average and resting heart rate, body mass, and sleep duration, and HealthExport.dailySummaries(calendar:) groups the parsed records by start-of-day and returns summaries sorted by day.

Part of #6. Closes #32.

How it was verified:

• mise run lint - no violations in the added files. Repo total stays at the pre-existing baseline (503 violations, 11 serious across 126 files, all in existing files). The lint task reports failure on that baseline, same as on origin/main before this change.
• mise run build - passed.
• mise run test - passed, 133 tests with 0 failures. The 9 new DailyHealthSummaryTests 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 aggregation logic only.

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

Test Case '-[BetterFitTests.DailyHealthSummaryTests testAverageHeartRateAndLatestRestingAndMass]' passed.
Test Case '-[BetterFitTests.DailyHealthSummaryTests testCodableRoundTrip]' passed.
Test Case '-[BetterFitTests.DailyHealthSummaryTests testDailySummariesFromParsedExport]' passed.
Test Case '-[BetterFitTests.DailyHealthSummaryTests testEmptyExportReturnsNoSummaries]' passed.
Test Case '-[BetterFitTests.DailyHealthSummaryTests testGroupsRecordsByDayAndSorts]' passed.
Test Case '-[BetterFitTests.DailyHealthSummaryTests testMissingKindsStayNil]' passed.
Test Case '-[BetterFitTests.DailyHealthSummaryTests testRecordsWithoutStartDateAreSkipped]' passed.
Test Case '-[BetterFitTests.DailyHealthSummaryTests testSleepDurationSumsStagesStartingThatDay]' passed.
Test Case '-[BetterFitTests.DailyHealthSummaryTests testSumsStepsEnergyAndDistanceWithNormalisation]' passed.
Executed 133 tests, with 0 failures (0 unexpected)

Each test name states what it shows. Together they cover grouping and sorting by day, summing steps with kilojoule and kilometre normalisation, averaging heart rate, picking the latest resting heart rate and body mass of the day, summing sleep stages, keeping missing kinds nil instead of zero, skipping records without a start date, an empty export, an end-to-end run through HealthExportParser, and a Codable round-trip.

Reviewer notes:

• Deliberately left out: CSV or JSON file writing to disk, and feeding summaries into recommendations. Those remain on Build Apple Health export ingestion and analysis pipeline #6 for later slices.
• Sleep is grouped by the day the sleep record starts, so a night crossing midnight counts toward its start day.
• Fields with no records that day stay nil rather than reading as a measured zero.
• Parent epic Build Apple Health export ingestion and analysis pipeline #6 stays open. This closes only the child slice Add daily health summaries aggregated from HealthRecord #32.

Follow-up verification (2026-11-08, later run):

• Re-checked locally on the branch tip (503defc, already on origin/main base f74c050, no rebase needed): mise run lint still 503 violations with none in the two added files, mise run build passed, mise run test passed with all 9 DailyHealthSummaryTests green.
• CI is now fully green. The first Mobile e2e (mobilewright) run failed one test with "app did not come to foreground" plus a WebSocket close, a simulator flake unrelated to this core-only change. Re-running the failed job passed (11m7s), and SwiftPM, iOS xcodebuild, iOS UI tests plus coverage, CodeQL, and Socket checks all pass. Merge state is CLEAN.
• Nothing user-facing changed, so no screenshot evidence applies. This PR is ready for human review and approval.