Recover continent membership and territory shapes by segmenting map images

The catalog reports `num_regions` per map (6 for World Classic, up to 36 for Land of Rus'), so
the count is known — the per-territory assignment is not. Continent bonuses are central to Risk
strategy, so the bonus-weighted-income feature in #1's position-strength model is blocked without
this.

## Approach

Continents are colour-coded on the map artwork itself. Drive the map selection panel with
Playwright, capture each map's image, and segment it.

- `/maps` is **robots-allowed**, as are `/assets/img/maps/.large.jpg` and
`/image/map/.large.circles.jpg`. The `.circles` variant overlays territory markers.
- Topology, by contrast, only exists on `/game/`, which robots.txt disallows — so the image
route is the *better-founded* path, not merely a fallback.
- This runs **once**, over 77 maps, not on any repeating schedule.

Segmentation should yield more than continents:

1. **Continent membership** — cluster territories by the artwork's fill colour at each known
`data-x`/`data-y`, or by segmenting regions directly.
2. **Territory polygons** — actual boundaries rather than just label points. This is what #1 asks
for when it says "high quality vector (plottable at any resolution) with labeled territories
and connections"; the current renderer draws nodes at label coordinates, which is a weaker
substitute.
3. **Adjacency, as a cross-check** — polygons that share a boundary are neighbours.

## Validation is available, and should gate this

`tests/fixtures/game_map1_territories.html` holds ground truth for World Classic straight from
D12: 42 territories, 83 undirected edges, exact adjacency, real names. Segmentation-derived
adjacency must be checked against it rather than trusted. If the pipeline cannot reproduce those
83 edges, it is not ready to run on the other 76 maps.

Report per-map agreement rather than a single aggregate — a method that works on World Classic and
fails on dense 150-territory maps needs to be known as such, not averaged into looking fine.

## Notes

- Map artwork is D12's and trademarked. Derived data (adjacency, membership, polygons) is factual
and fine to publish; **the source JPEGs are not to be redistributed**, and figures must render
our own geometry rather than reproducing their images.
- `region_id` already defaults to 0 throughout the codebase, and `check_invariants` treats a
wholly region-less map as valid rather than an error (map 77, "Brecourt Manor", genuinely has 0
regions). So this issue is additive — nothing needs restructuring to accept its output.

## Depends on