Smooth jagged sub-national region boundaries (v0.16.0)

Part of #16

scripts/regions.test.ts

@@ -404,4 +404,40 @@ describe('regions.geo.json', () => {
         ` / max ${maxId} ${(100 * maxRatio).toFixed(1)}% (band 40–175%)`,
     );
   });
+
+  it('is at the Douglas–Peucker 50m fixed point: 1,796 regions, topology unchanged, 111,175 vertices (v0.16 smoothing)', () => {
+    // v0.16 (scripts/smooth-regions.mjs) thinned every ring with Douglas–Peucker at
+    // ~50 m. That removes redundant vertices only — it never moves a border, merges
+    // a part, or creates/removes a ring — so the topology signature is invariant
+    // (1,796 features, 5,026 parts, 5,027 rings, the single KZ-KZY hole) and the
+    // vertex total sits at the pass's fixed point (re-running changes nothing).
+    // Pinning it guards against an accidental re-coarsening or a different-tolerance
+    // re-smooth silently changing the shipped dataset.
+    const polysOf = (g: Geometry) => (g.type === 'Polygon' ? [g.coordinates] : g.coordinates);
+    let verts = 0,
+      rings = 0,
+      parts = 0,
+      holes = 0;
+    const holeIds: string[] = [];
+    for (const f of regions.features) {
+      const polys = polysOf(f.geometry);
+      parts += polys.length;
+      for (const poly of polys) {
+        rings += poly.length;
+        holes += Math.max(0, poly.length - 1);
+        if (poly.length > 1) holeIds.push(f.id);
+        for (const ring of poly) verts += ring.length;
+      }
+    }
+    expect(regions.features.length, 'region count').toBe(1796);
+    expect(verts, 'total vertices (incl. ring closure)').toBe(111_175); // was 111,891 pre-v0.16; 716 removed
+    expect(parts, 'parts').toBe(5026); // unchanged — no part merged/dropped
+    expect(rings, 'rings').toBe(5027); // unchanged — no ring added/dropped
+    expect(holes, 'holes').toBe(1); // the single KZ-KZY hole, unchanged
+    expect(holeIds, 'the one remaining hole is KZ-KZY').toEqual(['KZ-KZY']);
+    console.log(
+      `\n  v0.16 smoothing: ${regions.features.length} regions, ${parts} parts, ${rings} rings, ` +
+        `${holes} hole (KZ-KZY), ${verts} vertices (50m DP fixed point; 716 removed from 111,891) — topology unchanged`,
+    );
+  });
 });