[ryanbr]

The Swift half of #593, twinning the Android implementation in #596 — the pure policy is byte-identical on both platforms (feature-level parity). Independent of #596 (no file overlap); the two can merge in either order.

What it does

Same opt-in, default-OFF nudge as the Android PR: once per day, when the day's Effort reaches the low end of today's recovery-derived optimal strain band (#43), post an "optimal strain reached" notification.

• StrainTargetNotifier.StrainTargetPolicy — pure gate + copy, semantics identical to Android's StrainTargetPolicy: fires only when enabled, both day strain and target are known, strain ≥ target, not already fired today; a nil target (recovery calibrating) never fires. Copy is NOOP's own wording — not WHOOP's decompiled strings (same clean-room stance as Add opt-in "optimal strain reached" notification (#593) #596).
• onDayUpdate — the BatteryNotifier idiom: requestAuthorization() up front when the toggle is enabled, status-only check at fire time, notification id "strain-target". The persisted day marker advances only after an authorized post, matching the Android notifier (a notifications-denied day still notifies once re-enabled while the same day shows the reached target).
• BehaviorStore.strainTargetNudge — default OFF.
• AppModel — the existing repo.$days sink also evaluates the strain target off the resolved today-row (repo.today, the same logical-day resolution every dashboard surface uses); converts stored 0–100 Effort → 0–21 via the shipped UnitFormatter.effortValue(_, .whoop) and gates on CoupledView.optimalStrainRange(recovery:).lowerBound — the exact same formatter + band the Android call site reads.
• AutomationsView — a "Strain target" card next to Battery alerts. AutomationsView is shared, so the toggle is reachable on macOS and iOS (unlike the macOS-only NotificationSettings screen). Enabling it also evaluates immediately (the reevaluateIllness idiom), so flipping it on when today's target is already reached posts without waiting for the next refresh.
• Localizable.xcstrings — 6 new keys with de/es/fr translations (informal de/es, formal fr, matching the existing catalog tone; Int interpolation as %lld per catalog convention).

Platform-appropriate divergences (deliberate)

• Toggle lives in Automations (where battery/illness/inactivity live on Apple platforms) rather than a "Daily reports" section — Android's daily-reports settings screen has no Swift twin (Add confirmation dialog when ending an active activity #517 never landed on iOS), and Automations is the established home + is shared with iOS.
• Dedupe day persists in UserDefaults (behavior.strainTargetLastDay), the BatteryNotifier convention, vs SharedPreferences on Android. Same semantics.

Tests

StrandTests/StrainTargetPolicyTests mirrors the Android StrainTargetPolicyTest byte-for-byte — same 7 fixtures, same expectations, including the copy assertions that NOOP's wording is present and WHOOP's strings are absent.

Verification (honest)

• On Linux: swiftc -parse clean on all six touched files; Tools/i18n_audit.py --ci main → exit 0 (no new un-extracted literals, focus locales complete); xcstrings JSON validated.
• Needs macOS: everything here is app-target (Strand/, StrandTests/) — not covered by swift-packages CI. An xcodebuild -scheme Strand … test / app-build.yml run is required to confirm compile + the 7 policy tests, and an on-device smoke test for actual delivery. New files are picked up by XcodeGen's directory globs (no project.yml change needed).