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feat(route-builder): densify + elevate off-trail segments

With "snap to route" off, every waypoint pair shipped as a 2-point
linear segment with no `<ele>` anywhere — the page only enriched
elevations inside the snap path. The resulting GPX had a flat
altitude profile and the build script's gain/loss/min/max metrics
came out all zero.

Two changes:

* New `densifyLinearSegments` (default 25 m, matches Swisstopo's
  coarsest DTM) walks every 2-point segment and seeds intermediate
  vertices along the great circle. Snapped segments (already many
  points from BRouter) are left alone.
* Page reactor now runs the same Swisstopo elevation enrichment in
  the autoSnap-off path, so the GPX carries per-trkpt `<ele>` even
  for fully manual / cross-country routes.

Elevation source unchanged: Swisstopo profile.json (COMB → DTM2 →
DTM25 fallback) is already the highest-resolution provider for our
Swiss-coverage hikes; no point swapping.

Also unifies the snap-path's inline enrichment call into the same
helper so there's one elevation code path instead of two.

2026-05-19 10:40:24 +02:00