Route overlay from a GPX, packed alongside the map

[Chemaclass]

From the README roadmap. Show a planned route on the map, packed alongside it.

A route is a single polyline. The packer already knows how to project, simplify
and quantise one, so most of this is plumbing rather than new geometry code:
read a GPX, run it through the same geom path, and emit it as its own layer.

Shape

• --gpx route.gpx on the packer, emitted as a dedicated layer id above the
  road layers so it draws on top.
• A palette slot for it. That means a new layer id, which is invariant 2 in
  CLAUDE.md: classify.py's L_* and Palette.mc must move together, and
  preview.py parses the palette at runtime.
• No new format version if it is just another layer, which is the appeal of
  doing it this way.

Open question

Whether the route belongs in the map pack at all, or in Storage like a
waypoint (#8). Compiled in means repacking to change your route, which is a
poor fit for something you change per outing. Storage means a second data path
on the watch. The second is more useful and more work.

[Chemaclass]

Not attempted in the sweep of 2026-08-08. The body's open question is still
open, and it is the thing that decides the work, so a recommendation rather
than a half-build.

Put a route in Application.Storage, not in the pack. A route is something
you change per outing, and compiling one in means repacking and reinstalling to
walk somewhere else, which is not a usable loop. The map is compiled in because
it is large and static; a route is neither.

That also sidesteps invariant 2 entirely. A route in Storage needs no new layer
id, so classify.py, Palette.mc and preview.py's runtime parse of it all
stay put, and there is no format version to bump. It draws in the overlay, like
the position marker, above the map buffer, so it costs no render restart when
it changes.

What it does need, and what makes it a real piece of work rather than an edit:

• somewhere to put a GPX. There is no filesystem API, so it arrives the way a
  city does, over HTTP from a URL in the settings, which means a downloader and
  a progress view or reuse of CityDownloader.
• a simplification pass before storing. 8 KB per value and ~128 KB total, shared
  with whatever city is downloaded, and Downloadable cities discard 85% of their geometry; look for room inside the same budget #5 showed that budget is already ~91 KB
  full for Berlin.

Related, and worth doing first: #8, which needs the same "small user data in
Storage alongside a city" question answered.