docs: create roadmap (4 phases)

Phases:
1. Semantic Position Model: DATA-*, PERS-* (foundation)
2. Constraint Validation: CONS-* (rules engine)
3. Visual Flattening: FLAT-* (display bridge)
4. Drag Interaction: DRAG-* (UI layer)

All 23 v1 requirements mapped to phases.

.planning/STATE.md

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+# Project State: Itinerary Editor
+
+## Project Reference
+
+**Core Value:** Drag-and-drop that operates on semantic positions (day + sortOrder), not row indices - so user intent is preserved across data reloads.
+
+**Current Focus:** Beginning Phase 1 - Semantic Position Model
+
+## Current Position
+
+**Phase:** 1 - Semantic Position Model
+**Plan:** Not yet created
+**Status:** Awaiting plan creation
+
+```
+Progress: [----------] 0%
+Phase 1:  [----------] Not Started
+Phase 2:  [----------] Not Started
+Phase 3:  [----------] Not Started
+Phase 4:  [----------] Not Started
+```
+
+## Performance Metrics
+
+| Metric | Value |
+|--------|-------|
+| Total Requirements | 23 |
+| Completed | 0 |
+| Current Phase | 1 |
+| Plans Executed | 0 |
+
+## Accumulated Context
+
+### Key Decisions
+
+| Decision | Rationale | Phase |
+|----------|-----------|-------|
+| UITableView over SwiftUI List | SwiftUI drag-drop lacks insertion line precision | Pre-planning |
+| (day, sortOrder) position model | Row indices break on reload; semantic position is stable | Pre-planning |
+| Insertion lines (not zones) | User wants precise feedback on exact drop location | Pre-planning |
+| Invalid drops rejected (snap back) | Cleaner than auto-clamping; user knows what happened | Pre-planning |
+
+### Learned
+
+- Previous attempts failed due to row-based thinking instead of semantic positioning
+- Travel was incorrectly treated as structural ("travelBefore") instead of positional
+- Hard-coded flatten order ignoring sortOrder caused reload issues
+
+### TODOs
+
+None yet - awaiting Phase 1 planning.
+
+### Blockers
+
+None currently.
+
+## Session Continuity
+
+**Last Session:** Project initialized, roadmap created
+**Next Action:** Create plan for Phase 1
+
+### Context for Next Session
+
+Starting fresh. Phase 1 establishes the semantic position model that everything else depends on. Requirements DATA-01 through DATA-05 define the model structure, PERS-01 through PERS-03 define persistence behavior. The key insight from research: row indices are lies, semantic positions are truth.
+
+---
+*State initialized: 2026-01-18*
+*Last updated: 2026-01-18*