Export the four ring metrics that stopped short of Apple Health

[ak710]

Closes #113

Merge order: 3 of 9. Branches off main, independent of #120 and #121.

What

Respiratory rate, VO₂max and blood glucose join the quantity path with the units HealthKit expects and the same plausibility bounds RingEventBridge already applies on the way into the store.

Blood pressure gets its own pass, because Health only recognises a reading when systolic and diastolic are saved together in an HKCorrelation — saved separately they are stored but never surface, which is indistinguishable from a silent failure.

Design notes

• The two BP halves are written from one packet at one instant, so that shared timestamp is the pairing key. A half without its partner is skipped rather than guessed at, and a pair with systolic ≤ diastolic is rejected as a misframed packet.
• The BP watermark reuses the bloodPressureSystolic slot in measurementWatermarks, which the quantity path never touches — so backfill and resetWatermarks keep working unchanged.
• Stress and fatigue stay unmapped, and the comment now says why properly. This isn't a follow-up: HealthKit has no type for a device-derived wellness score, and HKStateOfMind is a self-reported mood log that would misrepresent both. One existing test asserted these mapped to nil and needed updating for glucose — I left the two that are genuinely permanent.
• New toggles appear only for rings that can produce the metric, or that already have data for it. A jring owner never sees a VO₂max switch that couldn't write anything.

Testing

PulseLoopTests/HealthSyncNewTypesTests.swift — 11 tests on units, bounds, BP pairing and sync-ID stability. Full suite green (882).

Not verifiable in CI beyond the pure mappings — HKHealthStore isn't reachable from the test bundle, which is why this file follows the existing HealthSyncTests convention of testing the mapping tables only.