docs(progress): Add detailed P3 completion entry to changelog

Added comprehensive P3 completion entry covering:
- P3.1: Performance optimization & metrics
- P3.2: Enhanced observability
- P3.3: Developer experience improvements
- P3.4: Documentation polish

All items include verification status and implementation details.

docs/progress/01-CHANGELOG-WORK.md

@@ -11,6 +11,36 @@ For release notes, see [CHANGELOG.md](../../CHANGELOG.md).
 
 ## 2025-12-22
 
+### P3 Complete — Performance, Observability & Developer Experience
+
+- **2025-12-22 — P3.1 COMPLETE**: Performance optimization & metrics
+  - Created metrics contract infrastructure (`src/core/metrics.ts`) with `PerformanceMetric` interface, `MetricsCollector` interface, and `InMemoryMetricsCollector` class
+  - Instrumented recovery paths (Android `ReactivationManager.kt` + iOS `DailyNotificationReactivationManager.swift`) with timing
+  - Instrumented database operations (Android `ReactivationManager.kt`) with timing and slow query warnings (> 100ms)
+  - Created performance characteristics documentation (`docs/PERFORMANCE.md`) with expected performance benchmarks
+  - **Verification**: All instrumentation non-invasive, CI passes, performance docs linked in index
+- **2025-12-22 — P3.2 COMPLETE**: Enhanced observability
+  - Expanded event coverage: Added 9 new event codes (RECOVERY_START, RECOVERY_COMPLETE, RECOVERY_ERROR, DB_QUERY_START, DB_QUERY_COMPLETE, DB_QUERY_ERROR, STATE_TRANSITION, BACKGROUND_TASK_START, BACKGROUND_TASK_COMPLETE, BACKGROUND_TASK_ERROR)
+  - Implemented structured metrics export: `exportMetrics()` (JSON export) and `getMetricsSummary()` (lightweight summary)
+  - Enhanced error context: `logError()` method with structured error data including `DailyNotificationError` codes and stack traces
+  - Added opt-in diagnostic mode: `enableDiagnosticMode()`, `disableDiagnosticMode()`, `isDiagnosticMode()`, `getDiagnosticInfo()` methods
+  - Enhanced error serialization: Added `toJSON()` method to `DailyNotificationError` class
+  - **Verification**: All observability enhancements non-invasive, CI passes, no breaking changes
+- **2025-12-22 — P3.3 COMPLETE**: Developer experience improvements
+  - Enhanced error messages: Added `ERROR_GUIDANCE` constant with actionable guidance and platform hints for all error codes
+  - Added `NOT_SUPPORTED` error code for platform-specific operations
+  - Updated `web.ts` to use `DailyNotificationError` instead of plain `Error`
+  - Debug helpers: Added `getDebugState()` method to `web.ts` (throws NOT_SUPPORTED for web)
+  - Type tightening: Enhanced `ScheduleWithStatus` with `status` field ('active' | 'paused' | 'error')
+  - Integration examples: Created `docs/examples/QUICK_START.md` and `docs/examples/COMMON_PATTERNS.md`
+  - **Verification**: All changes non-breaking, CI passes, examples linked in index
+- **2025-12-22 — P3.4 COMPLETE**: Documentation polish
+  - Enhanced public API JSDoc: Improved documentation for `createSchedule()`, `updateSchedule()`, `deleteSchedule()`, `enableSchedule()` with parameter details, examples, and error documentation
+  - Created troubleshooting guide: `docs/TROUBLESHOOTING.md` covering CI failures, packaging, platform tests, build, permissions, recovery, performance
+  - Created getting started guide: `docs/GETTING_STARTED.md` with installation, platform setup, and basic usage
+  - Updated documentation index: Linked all new documentation in `docs/00-INDEX.md`
+  - **Verification**: All documentation follows established structure with drift guards, CI passes (except pre-existing TypeScript error in saveContentCache JSDoc)
+
 ### Changed
 - **2025-12-22 — P2.6 COMPLETE**: Type safety cleanup — eliminated all `any` usages except documented TypeScript mixin limitation
   - **Batch 1**: Replaced `any` return types in `src/vite-plugin.ts` with concrete types (`UserConfig`, `{ code: string; map: null }`)