test(plugin-chart-country-map): playwright e2e for static assets + end-to-end render

Two narrowly-scoped Playwright tests covering the integration layers
unit tests can't reach:

1. **serves all required static GeoJSON assets** — directly fetches
   each well-known file (manifest, default + ukr admin0, sample
   admin1, regional aggregation, composite) and asserts the server
   returns 200 + a parsable payload. Catches packaging regressions
   like the .dockerignore-drop that produced 404s on the PR-preview
   build despite the files being committed.

2. **renders a France subdivisions chart end-to-end** — creates a
   country_map_v2 chart via the API against the built-in
   birth_france_by_region dataset, opens its explore page, asserts
   the renderer's error banner ("Error loading map" / "No GeoJSON
   URL resolved") never appears, and counts at least 50 SVG path
   elements (one per French department). This is the canary the
   admin_level string/number bug and the all-black-render theme bug
   would have tripped.

Cross-viz form_data migration stays unit-tested in
migrateFromLegacy.test.ts; per-control visibility/validation stays
unit-tested in controlPanel.test.ts. This spec is reserved for
glue-layer checks only.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>