docs(01): capture phase context

Phase 01: Semantic Position Model
- Implementation decisions documented
- Phase boundary established

.planning/phases/01-semantic-position-model/01-CONTEXT.md

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+# Phase 1: Semantic Position Model - Context
+
+**Gathered:** 2026-01-18
+**Status:** Ready for planning
+
+<domain>
+## Phase Boundary
+
+All movable items have a persistent semantic position `(day: Int, sortOrder: Double)` that survives data reloads. Games are immovable anchors ordered by game time. Travel segments and custom items can be repositioned. This phase establishes the data model only — constraint validation, flattening, and drag interaction are separate phases.
+
+</domain>
+
+<decisions>
+## Implementation Decisions
+
+### sortOrder assignment
+- Use integer spacing (1, 2, 3...) for initial sortOrder values
+- Midpoint insertion (1.5, 1.25, etc.) when placing between items
+- Claude's discretion on threshold/rebalancing strategy when gaps get very small
+- Claude's discretion on negative vs positive sortOrder for "before games" items
+- Claude's discretion on Double vs Decimal storage type
+
+### Day boundaries
+- Day = trip day backed by calendar date (Day 1 = trip.startDate, Day 2 = startDate + 1, etc.)
+- Day number derived as: `calendar_days_since(trip.startDate) + 1`
+- Games belong to their start date, even if they run past midnight
+- Show all days including empty ones (no skipping gaps in the trip)
+
+### Travel segment identity
+- Each travel segment has a unique UUID (not keyed by route)
+- Same route (e.g., Chicago→Boston) can appear multiple times in a trip
+- Travel carries: from city, to city, estimated distance, estimated duration
+- Moving travel to different day just updates the day property (no recalculation)
+
+### Position initialization
+- Games get sortOrder assigned by Claude based on game time
+- Auto-generated travel appears after the origin city's games
+- Custom items added via "+" button on day headers, inserted at top of that day
+- Claude's discretion on handling position updates when trip is edited
+
+### Claude's Discretion
+- Exact sortOrder rebalancing threshold and strategy
+- Whether to use negative sortOrder or offset games to higher values for "before" positioning
+- Double vs Decimal for sortOrder storage
+- Initial sortOrder derivation for games (time-based or sequential)
+- Position preservation vs recomputation on trip edits
+
+</decisions>
+
+<specifics>
+## Specific Ideas
+
+- Day headers should have a "+" button for adding custom items
+- When user taps "+", item is added to top of that day (lowest sortOrder)
+
+</specifics>
+
+<deferred>
+## Deferred Ideas
+
+None — discussion stayed within phase scope
+
+</deferred>
+
+---
+
+*Phase: 01-semantic-position-model*
+*Context gathered: 2026-01-18*