Archive multi-day-routes change, sync spec to main

Synced delta spec (6 requirements) to openspec/specs/multi-day-routes/.
Archived change to openspec/changes/archive/2026-04-11-multi-day-routes/.

All 4 artifacts complete. All 35 tasks complete.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

openspec/specs/multi-day-routes/spec.md

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+## ADDED Requirements
+
+### Requirement: Overnight waypoint markers
+Any waypoint SHALL be toggleable as an overnight stop, creating day boundaries in the route.
+
+#### Scenario: Toggle overnight
+- **WHEN** a user toggles the overnight flag on a waypoint
+- **THEN** the waypoint's `overnight: true` flag is set in the Yjs document
+- **AND** the route is visually divided into days at that point
+
+#### Scenario: Implicit day boundaries
+- **WHEN** overnight stops are set
+- **THEN** the first waypoint is the implicit start of Day 1 and the last waypoint is the implicit end of the final day
+
+### Requirement: Per-day statistics
+The Planner SHALL compute and display distance, ascent, and estimated duration for each day.
+
+#### Scenario: Day stats computed
+- **WHEN** a route has overnight waypoints
+- **THEN** per-day distance, total ascent, and estimated duration are derived from segment boundaries and coordinates
+
+### Requirement: Day-aware sidebar
+The sidebar SHALL group waypoints by day with collapsible sections and per-day stats.
+
+#### Scenario: Day breakdown
+- **WHEN** a route has multiple days
+- **THEN** waypoints are grouped under "Day 1", "Day 2", etc. with collapsible sections
+- **AND** each section header shows day distance and ascent
+
+### Requirement: Elevation chart day dividers
+The elevation chart SHALL show day boundaries as dashed vertical lines.
+
+#### Scenario: Day dividers on chart
+- **WHEN** a route has multiple days
+- **THEN** dashed vertical lines with "Day N" labels appear at each overnight waypoint position
+
+### Requirement: Map day labels
+The map SHALL display day summary labels at day boundary waypoints.
+
+#### Scenario: Day labels on map
+- **WHEN** a route has multiple days
+- **THEN** white pill markers at day boundaries show "Day N . X km"
+
+### Requirement: Multi-day GPX export
+Day structure SHALL be preserved in GPX exports via waypoint type elements.
+
+#### Scenario: Export multi-day route
+- **WHEN** a user exports a plan with overnight waypoints
+- **THEN** overnight waypoints include a `<type>overnight</type>` element in the GPX
+- **AND** reimporting the GPX restores the day structure