feat(country-map): scaffold modern plugin shell

Bootstrap the new @superset-ui/plugin-chart-country-map package with
the modern ChartPlugin pattern. Mirrors plugin-chart-handlebars'
structure (the leanest existing modern plugin in the repo).

Files:
  package.json                     — workspace pkg with d3-* deps
  tsconfig.json                    — extends frontend root tsconfig,
                                     references core/chart-controls
  types/external.d.ts              — ambient types for png/jpg/geojson
  README.md                        — what this plugin is + status
  src/
    index.ts                       — package entry
    types.ts                       — CountryMapFormData + transform props
    CountryMap.tsx                 — placeholder renderer (real D3 port
                                     in a follow-up commit)
    plugin/
      index.ts                     — ChartPlugin class with metadata
      buildQuery.ts                — modern chart/data query (this is
                                     the core difference from the legacy
                                     plugin's explore_json path)
      controlPanel.tsx             — minimal placeholder; full control
                                     set (worldview / admin level /
                                     country / region include-exclude /
                                     fly-islands / language) ports next
      transformProps.ts            — derives geoJsonUrl from form_data
                                     using the build-script's output
                                     naming convention

Renderer scaffold fetches the resolved GeoJSON URL and shows a small
diagnostic block (URL + feature count + data row count + metric name)
so the plugin compiles end-to-end and can be wired to the chart-type
registry without a real D3 render. Real renderer + controls land in
the next commits.

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

superset-frontend/plugins/plugin-chart-country-map/src/plugin/transformProps.ts

@@ -0,0 +1,72 @@
+/**
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+import {
+  CountryMapChartProps,
+  CountryMapTransformedProps,
+} from '../types';
+
+/**
+ * Translate Superset's standard ChartProps into the shape the renderer
+ * needs. Notable: derive `geoJsonUrl` from the form data so the renderer
+ * can fetch the right output from the build pipeline.
+ *
+ * URL layout (matches the build script's output naming):
+ *   <worldview>_admin0.geo.json                — world choropleth
+ *   <worldview>_admin1_<adm0_a3>.geo.json      — country subdivisions
+ *   regional_<adm0_a3>_<set>_<worldview>.geo.json — aggregated regions
+ *   composite_<id>_<worldview>.geo.json         — composite maps
+ *
+ * The actual hosting path is wired up in a follow-up commit; for now
+ * the renderer prefixes URLs with a stubbed base.
+ */
+const GEOJSON_BASE = '/static/assets/country-maps';
+
+export default function transformProps(
+  chartProps: CountryMapChartProps,
+): CountryMapTransformedProps {
+  const { formData, queriesData, width, height } = chartProps;
+  const data = (queriesData?.[0]?.data as Record<string, unknown>[]) ?? [];
+
+  const worldview = formData.worldview || 'ukr';
+  const adminLevel = formData.admin_level ?? 0;
+
+  let geoJsonUrl: string | null = null;
+  if (formData.composite) {
+    geoJsonUrl = `${GEOJSON_BASE}/composite_${formData.composite}_${worldview}.geo.json`;
+  } else if (formData.region_set && formData.country) {
+    geoJsonUrl =
+      `${GEOJSON_BASE}/regional_${formData.country}_${formData.region_set}_${worldview}.geo.json`;
+  } else if (adminLevel === 1 && formData.country) {
+    geoJsonUrl =
+      `${GEOJSON_BASE}/${worldview}_admin1_${formData.country}.geo.json`;
+  } else if (adminLevel === 0) {
+    geoJsonUrl = `${GEOJSON_BASE}/${worldview}_admin0.geo.json`;
+  }
+
+  return {
+    width,
+    height,
+    formData,
+    data,
+    geoJsonUrl,
+    metricName: typeof formData.metric === 'string' ? formData.metric : null,
+    numberFormat: formData.number_format,
+    linearColorScheme: formData.linear_color_scheme,
+  };
+}