# V3-09 — Follow a planned route while riding

**Phase** Route planning · **Depends on** V3-04, V3-08 (itself blocked on V3-17) · **Size** M · **Status** Deferred

## Goal
Pick a saved route before starting, see it on the live map underneath your actual track,
and afterwards compare what you rode against what you planned.

## Context
The payoff that makes V3-07 and V3-08 worth building, and the point where route planning
meets the live map. Not navigation — no turn-by-turn, no voice. Just the line you meant to
follow, drawn under the line you actually rode.

## Design
Attach an optional `routeId` to `Trip`. That is enough for both the live overlay and the
after-the-fact comparison.

Live: the planned route in a muted colour, the recorded track drawn over it in the existing
speed colours. Immediately obvious when you have left the plan.

Afterwards, on trip detail: both lines, plus how far you deviated and how the real duration
compared with the estimate — which also, over time, tells you how honest the ETA is.

**Deliberately not:** rerouting, off-route alerts, or anything that demands attention while
riding. A rider glancing at handlebars wants a picture, not an interruption.

## Implementation
1. `routeId` on `Trip` — **third migration**
2. Route picker on the record screen before Start, defaulting to none
3. Live map renders the planned polyline beneath the track
4. Trip detail renders both, with a comparison block
5. Deviation: max and mean distance from the recorded points to the planned polyline —
   `perpendicularDistanceMeters` already exists and is parity-proven

## Acceptance criteria
- [ ] A route can be selected before starting, or not
- [ ] Both lines render, visually distinguishable
- [ ] Deviation and duration-vs-estimate appear on trip detail
- [ ] A ride with no route behaves exactly as today
- [ ] Deleting a route does not delete rides that referenced it

## Tests
- Deviation maths against a known track and route
- Repository: deleting a route nulls `routeId` rather than cascading to the trip —
  **the cascade direction here is the opposite of segments and is easy to get wrong**
- Widget: both polylines present when a route is attached

## Risks
The `Route` → `Trip` foreign key must **not** cascade. Deleting an old plan must never
delete the ride you did.

## Out of scope
Turn-by-turn, off-route alerts, rerouting.