docs: add comprehensive product roadmap (5 phases, Option D strategy)
- Phase 1: Web MVP (Next.js 16 PWA) — timer engine, cascade, visual timeline
- Phase 2: Intelligence — routines, NL input, focus mode, neurodivergent mode
- Phase 3: Apple Native — iOS + Apple Watch + macOS (SwiftUI, not KMP)
- Phase 4: Context & Growth — calendar sync, location, sleep, viral mechanics
- Phase 5: Android + Scale — Jetpack Compose, Wear OS, shared timers, monetization
- Includes: viral playbook, success metrics, risk register, tech stack summary
- Checklists for every phase with exit criteria
c02d8cf08c