docs: document DaytimeStress, StressIndex, HRVFreqDomain, StressOnsetDetector

[vishk23]

Summary

docs/ANALYTICS.md's "What is actually wired into the app" section omitted the Stress screen's live analytics entirely: DaytimeStress, StressIndex, HRVFreqDomain, and StressOnsetDetector had zero mention despite all being wired and live.

• DaytimeStress (Packages/StrandAnalytics/Sources/StrandAnalytics/DaytimeStress.swift) — buckets waking hours (06:00-22:00) from the day's hrSample + rrInterval streams, z-scores hourly HR-elevation / RMSSD-suppression against the day's own calm hours (not a personal baseline), and squashes onto the same 0-3 logistic the daily Stress score uses. Day-relative today; documented as such without promising a future baseline-relative mode.
• StressIndex (Baevsky Stress Index) and HRVFreqDomain (LF/HF) — two additive, on-demand HRV lenses, purely informational (don't feed the 0-3 score).
• StressOnsetDetector — the L3 edge-triggered, exercise-gated stress check-in, opt-in and default-off.

Wired call sites confirmed by reading source

• DaytimeStress.analyze(hr:rr:tzOffsetSeconds:) <- Strand/Screens/StressView.swift loadDaytime(), rendering the "Today's Timeline" card (DaytimeLoadLine, StressTotalsBar) and the sustained-high Breathe-suggestion card. Reachable on iOS via More -> Stress (StrandiOS/App/RootTabView.swift, .stress -> StressView()).
• StressIndex.components(rr:) / StressIndex.stressIndex(rr:) <- StressView.loadDaytime() (Advanced HRV card) and Strand/AI/AICoach.swift's stressIndexLine() (folded into the AI Coach context, consent-gated).
• HRVFreqDomain.freqDomain(rr:) <- the same StressView.loadDaytime() call as StressIndex — found wired-but-undocumented alongside it during this pass (not explicitly asked for, flagging it since it shares the exact call site).
• StressOnsetDetector.evaluate(...) <- Strand/App/AppModel.swift's evaluateStress(), gated behind a default-off "stress check-ins" toggle (BiofeedbackPrefs.stressConfig()), state persisted via BiofeedbackPrefs, surfaced via StressCheckInCard/StressNudgeCenter.

Test plan

• Read docs/ANALYTICS.md, DaytimeStress.swift, StressView.swift, StressIndex.swift, StressOnsetDetector.swift, RootTabView.swift, AppModel.swift, AICoach.swift, BiofeedbackPrefs.swift, StressCheckInCard.swift in full.
• Grepped every call site of DaytimeStress., StressIndex., HRVFreqDomain., StressOnsetDetector. across Strand, StrandiOS, Packages to confirm nothing else was missed and nothing claimed here is speculative.
• Docs-only change; no source touched.

[vishk23]

Merged current main (297 commits) and re-verified every claim against the code as it stands today — worth doing, because part of this PR had gone stale.

What dropped out on its own. The 3-way merge kept upstream's later edits to the two regions this branch originally rewrote, so two now-wrong pieces disappeared without my touching them:

• the HRVAnalyzer table-row edit (it claimed the app computes RMSSD inline via AppModel.rmssd(_:); upstream's row — "Live stress check-ins use the dedicated StressOnsetDetector" — is the correct one and survives);
• the whole "RMSSD for the stress nudge" section, which documented an AppModel.rmssd and a behavior.stressNudge toggle that no longer exist.

One correction I made by hand. This branch's own text said StressOnsetDetector runs "alongside (not replacing) the legacy inline nudge". That is no longer true — AppModel.evaluateStress() is now the detector's only caller and the legacy inline nudge is gone. Reworded, and the gate list corrected to cfg.enabled + autoNudge + live.bonded + live.worn.

Re-verified as still accurate on current main: StressView.loadDaytime() still calls DaytimeStress.analyze(hr:rr:tzOffsetSeconds:), StressIndex.components(rr:) and HRVFreqDomain.freqDomain(rr:); AICoach.stressIndexLine() still exists; BiofeedbackPrefs.stressConfig() still carries the two default-off toggles. All four engine files are still present and still undocumented in ANALYTICS.md, so the gap this PR fills is real.

Docs-only, 1 file. Two notes for whoever lands it:

• The ### 1. / ### 2. / ### 4. numbering gap in "Live analytics in AppModel" is pre-existing on main (left by the removal of the old section 3) — deliberately not touched here, one concern per PR.
• If Daytime stress: score against a personal cross-day baseline, not the day's own calm hours #463 lands first, the DaytimeStress row's "day-relative — not the personal baseline" wording becomes stale, since that PR makes exactly that change. Cheapest order is docs last, or I can fold the update in.