docs(issues): KC-51 — analytics system plan, persistence eval, blockers

Records the /goal analytics-system implementation plan: reusable-functionality
survey, SwiftData/CoreData/current persistence evaluation, proposed architecture
(stores, immutable reports, pure analysis engine, reconciliation, notifications,
UI, tests), and the two blockers (build-verification unavailable; persistence +
min-iOS decision).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

KisaniCal/ISSUES.md

@@ -1451,3 +1451,85 @@ toggle, a Snooze submenu (15 min / 1 hr / tomorrow), then the shared
 Events (no `taskID`) get no menu.
 
 Files: `CalendarView.swift`.
+
+---
+
+## KC-51 — Health & Workout Analytics system (12-month, immutable reports) — PLAN
+
+**Status:** Planning (blocked on 2 decisions + build-verification environment)
+**Reported by:** User (/goal)
+**Area:** Analytics / Persistence / HealthKit / Notifications
+
+### Reusable existing functionality (survey)
+- `WorkoutViewModel`: `history: [WorkoutDayLog]`, `workoutDates`, `schedule`,
+  `missedDates`, `compensations`; `StatPeriod`/`WorkoutStat`/`StatBucket`
+  aggregation already exists (`stat(period:offset:)`, `statBuckets`).
+- `WorkoutDayLog` → `LoggedExercise` → `LoggedSet(weight, reps, done)` with
+  computed `volume` (Σ weight×reps for done sets), `totalSets`, `doneSets`.
+- `HealthKitManager`: steps, active calories, resting HR, workoutsThisWeek,
+  `fetchWorkoutDates(from:to:)`, `saveWorkout`, `sumQuery`/`latestQuery`.
+- `NotificationManager`: categories/actions, foreground `reschedule(...)`, and
+  **`schedulePeriodMilestones()`** (week/month/year-close nudges) — extend this.
+- Lifecycle reconcile pattern already used: `checkPendingMissed` /
+  `checkPendingHealthConfirm` on `onAppear` + `scenePhase == .active`.
+- Persistence: `UserDefaults` mirrored to iCloud KVS (`CloudSyncManager`).
+
+### Persistence evaluation (SwiftData / Core Data / current)
+- **Current (JSON blobs in UserDefaults + iCloud KVS):** iCloud KVS has a hard
+  **1 MB total** ceiling (see KC-39 context). 12 months of daily snapshots +
+  HealthKit reference cache **will exceed it** → silent sync failure. Not viable
+  as the analytics store.
+- **SwiftData:** cleanest modelling/migration, but requires **iOS 17+**. App
+  deploymentTarget is **iOS 16.0** → SwiftData is UNAVAILABLE unless the min
+  target is raised (drops iOS 16 devices — a product call).
+- **Core Data:** works at iOS 16, unbounded local storage, supports lightweight
+  migration; more boilerplate. Local-only (analytics need not sync via KVS;
+  reports are device-derived and reconstructable).
+
+### Proposed architecture (pending the decision below)
+- **Stores:** `WorkoutRecordStore` (app-generated, mirrors `WorkoutDayLog`) +
+  `HealthReferenceStore` (HealthKit cache; HealthKit = source of truth) +
+  `ReportStore` (immutable daily snapshots, weekly, monthly). Keep app records
+  and HealthKit reference separate; analysis reads across both.
+- **Immutability:** reports store a frozen snapshot at generation; later edits to
+  source data never mutate a generated report. Retention: prune raw beyond 12
+  months; keep report summaries.
+- **Analysis engine (`AnalyticsEngine`)**: pure, no SwiftUI/persistence deps,
+  deterministic. Formulas documented: volume = Σ(weight×reps) of completed sets;
+  %Δ = (cur−base)/|base| guarded for base≈0/new/rest; consistency = sessions ÷
+  scheduled over window; trend = classify(slope, threshold) → improve / decline /
+  plateau / insufficient-data. Handles missing/partial HK, deloads, outliers
+  (winsorize/IQR), unit compatibility, divide-by-zero. Non-medical language +
+  disclaimer; never diagnoses.
+- **Comparisons:** daily vs previous comparable day + same weekday −1 week;
+  weekly vs previous completed week; monthly vs previous month.
+- **Reconciliation:** idempotent generators keyed by period id; on launch/active,
+  backfill any missing completed week/month; never duplicate reports or
+  notifications. Do not depend on exact BG execution.
+- **Notifications:** extend `schedulePeriodMilestones()`; one permission-aware,
+  user-scheduled local notification per weekly/monthly report; deep-link via
+  `userInfo` → route to the specific report (new deep-link cases).
+- **UI (`Analytics` area):** Overview · Daily comparison · Weekly reports ·
+  Monthly reports · 12-month trends · Per-exercise history. Full loading / empty /
+  denied-permission / partial-data / error states; Dynamic Type, VoiceOver,
+  light/dark, compact/regular; Wenza design tokens; charts only where they aid
+  understanding; non-color status indicators.
+- **Tests:** date boundaries (daily/weekly/monthly), calendar/locale/firstWeekday/
+  timezone/DST, volume/%/consistency/trend math, missing/partial HK, migration/
+  dedup/reconcile/retention, exercise identity + unit compat, report+notification
+  idempotency, deep-link routing.
+
+### BLOCKERS (must resolve before build-verified completion)
+1. **Build/test verification is impossible in this environment.** The iOS 26.5
+   simulator runtime was removed this session and CLI builds are broken, so
+   completion conditions #10 (tests pass) and #11 (Xcode build succeeds) CANNOT
+   be verified here. Per "never claim verification not performed", any code
+   produced now is inspection-only until a runtime is restored / built on device.
+2. **Persistence + min-iOS decision (irreversible):** SwiftData needs iOS 17
+   (app is iOS 16). Migrating live workout history to a new store is a one-way
+   data migration. Needs the founder's call before implementation.
+
+### Next action
+Awaiting decision on persistence backend / deployment target, then implement the
+pure `AnalyticsEngine` + models + tests first (most inspection-verifiable), UI
+last. Device/build QA tracked here.