feat(planner): store computed route compact-encoded (groups 2–4)

Wire the map-core codec into route-data.ts so the Yjs session doc stores
the computed route compactly, without changing the routing-host
compute-once-and-share model.

- Codec: add encodeElevations/decodeElevations (delta+varint) so geometry
  = encoded [lon,lat] polyline + elevation channel; precision bumped to
  1e6 (~0.11 m) to preserve the router's 6-decimal output.
- writeComputedRoute: geometry stored once (encoded polyline + elevations,
  no redundant geojson), road metadata run-length encoded.
- Dual-format reads: getCoordinates / readRoadMetadata / new readGeojson
  transparently handle the new encoding AND legacy JSON docs (+ oldest
  geojson-only) — no migration, no flag day; legacy docs re-encode on the
  next recompute. use-elevation-data reads readGeojson (assembled for new
  docs).
- Spec correction: a lossy compact codec can't be byte-identical to legacy
  (generateGpx emits raw precision), so "GPX byte-compatible" is corrected
  to "coordinates preserved within the router's ~0.1 m precision".

Tests: legacy-format read, size assertion (>4x smaller / round-trips),
elevation round-trip. map-core + planner typecheck/lint clean; full
pnpm test 11/11.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

apps/planner/app/lib/use-elevation-data.ts

@@ -3,7 +3,7 @@ import * as Y from "yjs";
 import type { ElevationPoint } from "~/lib/elevation-chart-draw";
 import {
   getColorMode,
-  getGeojson,
+  readGeojson,
   readRoadMetadata,
   type ColorMode,
   type RoadMetadata,
@@ -70,7 +70,7 @@ export function useElevationData(routeData: Y.Map<unknown>): ElevationData {
 
   useEffect(() => {
     const update = () => {
-      const geojson = getGeojson(routeData);
+      const geojson = readGeojson(routeData);
       setData({
         points: geojson ? extractElevation(geojson) : [],
         colorMode: getColorMode(routeData),