fix(health): feed the Android body clock, closing a live cross-platform gap

[ryanbr]

Closes #852. Supersedes #853, which I closed — that PR proposed deleting this, on the mistaken belief there was no Swift twin.

The gap

Body Clock has shipped on iOS/macOS all along. AppModel.computeCircadianPhase pools HR by local hour-of-day as the activity proxy, fits the cosinor, publishes circadianPhase, and SkinTempCardsView renders it.

Android has a byte-for-byte twin of the engine, its own full test suite, and a finished BodyClockCard — and never fed any of it. V5HealthSignals hardcoded bodyClock = null, so HealthScreen's ?.let skipped the card on every device.

The comment guarding it was wrong. It said we lack "per-hour rest-activity bins we don't bank here". Swift doesn't bank them either — it derives them from HR buckets Android also has. The pipe was never built on this side, not missing.

The change

Two hunks, ~30 lines. Mirrors the Swift maths exactly: 3600 s buckets over 14 days, the >= 24-bucket floor, pooling by local hour, mean bpm per populated hour, distinct local days as daysObserved, and the >= 6-bin floor before a fit is attempted.

The binning lives in AppViewModel, not CircadianEngine, because Swift keeps it in AppModel — the engine stays an untouched byte-for-byte mirror on both platforms, which is the point.

Its pure half is a top-level circadianBinsFrom so it is testable with no Context, no ViewModel and no database.

Verification

Six tests pin the floors, local-hour pooling, mean-not-sum, the negative-offset wrap and sparse coverage. Every expectation was cross-checked against an independent implementation of the algorithm before commit:

OK under24     bins=0  days=0      OK plus1h48   bins=24 days=3
OK exactly24   bins=24 days=1      OK neg5h      bins=24 (hours stay 0..23)
OK meanNotSum  bins=24 mean=60.0   OK sparse6x5  bins=6  days=5

Both new parameters default, so every existing evaluate() caller is byte-identical. The synchronous settings-toggle re-evaluate reuses the collector pass's cached bins rather than blanking the estimate — it has no coroutine to read the store from.

Known divergence, deliberately not closed here

Swift derives habitualWakeHour from the most recent banked wake, falling back to 07:00. Kotlin has no such helper, so this uses the existing 7.0 default. That shifts offsetVsScheduleMinutes for anyone who doesn't wake at 07:00 — real and bounded, worth a follow-up, but a wake-time derivation is a separate change.

What this does NOT settle

Whether this should ship at all. Without an observedTempMinHour, estimatePhase derives the CBT minimum as activityAcrophase − 12.0h — a population constant — and the confidence bands key on data sufficiency, not on whether that constant holds for this person.

That question is about the already-shipped Apple card. Leaving Android silently inert was never an answer to it, which is why this lands first: it makes the platforms consistent so the real decision can be made once, for both.

Not compiled — android.yml is disabled.

[ryanbr]

Cleared the two nits from the last pass, both cosmetic:

• java.util.TimeZone now sorts before kotlin.math.roundToInt, matching the java-then-kotlin order used elsewhere in the tree (HealthConnectImporter, DataBackup).
• Stopped round-tripping the clock — it divided currentTimeMillis() to seconds and multiplied back to look up the timezone offset. Keeps nowMs and uses it directly. No behaviour change; the offset is identical anywhere inside the same second.

Also documented, from the pass before: lastCircadianBins is a plain var written from a coroutine and read from a UI callback, which reads as a race and is not one — the write resumes on viewModelScope (Dispatchers.Main.immediate) and the read is a main-thread settings callback. The Swift twin gets the same guarantee from @MainActor. Recorded so the next reader does not have to re-derive it.

Both gates green. Nothing else outstanding from my side — the two limitations in the description (habitualWakeHour defaulting to 7.0, and the unresolved population-constant question) are deliberate and stated, not oversights.