openspec: propose route-surface-breakdown

Surface + waytype proportion bars on route detail, from the BRouter waytags the
Planner already computes (and currently discards on save). Design records the
data-source decision: persist a compact distance-weighted breakdown for
Planner-created routes (accurate, no external call); Overpass map-matching for
imports/uploads is parked as a future option.

Validates with `openspec validate --strict`. Specs only; no code.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

openspec/changes/route-surface-breakdown/proposal.md

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+## Why
+
+"How much of this is gravel?" is a top-three question for anyone choosing a
+route — Komoot's surface/waytype breakdown bars are one of its most-used
+features, and `@trails-cool/map-core` already ships the surface/highway colour
+palettes. We also already *compute* accurate per-segment surface and highway
+tags from BRouter while planning (`route-merge` → `EnrichedRoute.surfaces[]`),
+then **throw them away on save**. This change captures that data and shows it.
+
+## What Changes
+
+- On **route detail**, show surface and waytype **proportion bars** (e.g.
+  "62% asphalt · 28% gravel · 10% path"), coloured with the `map-core`
+  `SURFACE_COLORS` / `HIGHWAY_COLORS` palettes, with per-segment km/percent.
+- **Persist the breakdown for Planner-created routes**: compute a
+  distance-weighted surface + waytype distribution from the BRouter waytags at
+  save time, send it in the planner→journal handoff, and store it on the route.
+- Degrade gracefully: routes without breakdown data (older routes, or activities
+  / manually-imported GPX that never had waytags) simply don't show the bars.
+
+## Capabilities
+
+### New Capabilities
+
+- `route-surface-breakdown`: the surface/waytype proportion bars on route
+  detail, how the distribution is derived from BRouter waytags, persisted at
+  save, and rendered.
+
+### Modified Capabilities
+
+<!-- `planner-journal-handoff` gains one optional field in its save payload;
+     noted in design, but the breakdown is a new capability, not a change to the
+     handoff's existing requirements. -->
+
+## Impact
+
+- **Schema**: a nullable `surface_breakdown` jsonb column on `journal.routes`
+  (additive — `pnpm db:push`, no data migration).
+- **Planner**: `api.save-to-journal` computes the distance-weighted breakdown
+  from the session `EnrichedRoute` (surfaces/highways + coordinates) and adds it
+  to the POST body.
+- **Journal**: the route callback (`api.routes.$id.callback`) accepts + stores
+  it; the route detail loader exposes it; a `SurfaceBreakdown` component renders
+  the bars (reusing `map-core` palettes + `stats.ts` distance formatting).
+- **No new external dependency** — BRouter already returns the tags; no Overpass
+  call. Imported activities/uploads are out of scope for v1 (see design).