docs: create roadmap (4 phases)

Phases:
1. Foundation: code organization, model extraction (ORG-01–08)
2. Provider migration: setState → ChangeNotifier/Consumer (STATE-01–07)
3. Overlay refactor: fix data staleness, error surfacing (OVRL-01–06)
4. GPS & power: adaptive precision, lifecycle management (GPS-01–04, PWR-01–06)

All v1 requirements mapped to phases.

.planning/STATE.md

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+# Project State
+
+## Project Reference
+
+See: .planning/PROJECT.md (updated 2026-02-09)
+
+**Core value:** Reliable, lag-free GPS speed display that coexists politely with other Android apps
+**Current focus:** Phase 1 - Foundation
+
+## Current Position
+
+Phase: 1 of 4 (Foundation)
+Plan: Not yet planned
+Status: Ready to plan
+Last activity: 2026-02-09 — Roadmap created
+
+Progress: [░░░░░░░░░░] 0%
+
+## Performance Metrics
+
+**Velocity:**
+- Total plans completed: 0
+- Average duration: N/A
+- Total execution time: 0.0 hours
+
+**By Phase:**
+
+| Phase | Plans | Total | Avg/Plan |
+|-------|-------|-------|----------|
+| - | - | - | - |
+
+**Recent Trend:**
+- Last 5 plans: None yet
+- Trend: N/A
+
+*Updated after each plan completion*
+
+## Accumulated Context
+
+### Decisions
+
+Decisions are logged in PROJECT.md Key Decisions table.
+Recent decisions affecting current work:
+
+- Architecture: Provider pattern for state management (pending implementation)
+- Architecture: Speed-adaptive GPS precision with ~10 km/h threshold (pending implementation)
+- Architecture: Background GPS only when overlay visible (pending implementation)
+- Architecture: Layer-first directory structure (screens/, widgets/, services/, providers/, models/) (pending implementation)
+- Architecture: No new dependencies beyond provider package (pending implementation)
+
+### Pending Todos
+
+None yet.
+
+### Blockers/Concerns
+
+**Phase 2 (Provider Migration):**
+- HIGH RISK: Async lifecycle complexity — 5+ async patterns must be audited before migration
+- Must create manual regression testing checklist before starting Phase 2
+- GpsDataManager singleton disposal strategy needs design decision
+
+**Phase 3 (Overlay Refactor):**
+- MEDIUM RISK: flutter_overlay_window 0.5.0 has known communication issues
+- May need testing on multiple Android versions for reliability
+- Overlay status polling alternative needs research (event-based vs 5s polling)
+
+**Phase 4 (GPS Optimization):**
+- Speed thresholds (8/12 km/h) need field testing validation
+- Android OEM behavior varies (Samsung, Xiaomi duty-cycling differences)
+
+## Session Continuity
+
+Last session: 2026-02-09 (roadmap creation)
+Stopped at: Roadmap and STATE files written, ready for Phase 1 planning
+Resume file: None