Build region-adjacency manifest (Stage 1 of #16 P2)

Generate public/region-adjacency.json: regionKey -> neighbourKeys, built
from regions.geo.json (bbox proximity reject + @turf/boolean-intersects
arbitration; symmetric, deduped, no self-refs). Deterministic + idempotent.

This is the data backing the LLM's getNeighbours tool (Stage 2) which lets
it move contiguous frontier regions instead of whole countries.

scripts/region-adjacency.test.ts

@@ -0,0 +1,87 @@
+/**
+ * scripts/region-adjacency.test.ts — validation for the committed
+ * `public/region-adjacency.json` (built by `scripts/build-region-adjacency.mjs`).
+ *
+ * The manifest maps each region key to the list of region keys it **borders**
+ * (shares an edge/point with). It is the data behind the model's `getNeighbours`
+ * tool (v0.21, #16 P2), so it is a load-bearing data artifact — this test pins
+ * its contract.
+ *
+ * Deliberately dependency-free: it reads the JSON with plain `fs`/`JSON.parse`
+ * and never imports `@turf`/geojson types (importing one pulls `@types/geojson`
+ * into the tsc program and breaks `WorldMap.tsx`; see the v0.17 changelog note).
+ * Mirrors the style of `scripts/region-keys.test.ts`.
+ */
+import { readFileSync } from 'node:fs';
+import path from 'node:path';
+import { fileURLToPath } from 'node:url';
+import { describe, expect, it } from 'vitest';
+
+const here = path.dirname(fileURLToPath(import.meta.url));
+const root = path.resolve(here, '..');
+
+function readJson(rel: string): Record<string, unknown> {
+  return JSON.parse(readFileSync(path.join(root, rel), 'utf8'));
+}
+
+describe('region-adjacency.json', () => {
+  const adjacency = readJson('public/region-adjacency.json') as Record<string, string[]>;
+  const manifest = readJson('public/region-keys.json') as Record<string, string>;
+  const keys = new Set(Object.values(manifest));
+
+  it('has exactly one entry per region key (same key set as region-keys.json)', () => {
+    const manifestKeys = new Set(Object.values(manifest));
+    const adjacencyKeys = new Set(Object.keys(adjacency));
+    expect(adjacencyKeys).toEqual(manifestKeys);
+    expect(Object.keys(adjacency)).toHaveLength(keys.size);
+    expect(adjacencyKeys.size).toBe(1796);
+  });
+
+  it('every neighbour is itself a valid region key (referential integrity)', () => {
+    for (const [key, neighbours] of Object.entries(adjacency)) {
+      for (const n of neighbours) {
+        expect(keys.has(n), `neighbour ${n} of ${key} is not a valid region key`).toBe(true);
+      }
+    }
+  });
+
+  it('no region lists itself as a neighbour', () => {
+    for (const [key, neighbours] of Object.entries(adjacency)) {
+      expect(neighbours.includes(key), `${key} lists itself`).toBe(false);
+    }
+  });
+
+  it('adjacency is symmetric (A ∈ B  ⟺  B ∈ A)', () => {
+    let checked = 0;
+    for (const [a, aList] of Object.entries(adjacency)) {
+      const aSet = new Set(aList);
+      for (const b of aList) {
+        const bList = adjacency[b];
+        expect(Array.isArray(bList), `${b} has no adjacency entry`).toBe(true);
+        // A lists B, so B must list A.
+        expect(bList!.includes(a), `${a} → ${b} but ${b} does not list ${a}`).toBe(true);
+        checked++;
+      }
+      // And the converse: every neighbour B of A must be a key (implied above) —
+      // also assert no *missing* reverse entry by checking the full set equality.
+      const bSet = new Set(aSet);
+      for (const b of aList) bSet.add(b);
+      // Symmetry means a's set equals the set of {b : a ∈ b}.
+      const reverse = Object.entries(adjacency)
+        .filter(([, list]) => list.includes(a))
+        .map(([k]) => k);
+      expect(new Set(reverse).size, `asymmetric degree for ${a}`).toBe(aList.length);
+      expect(new Set(aList).size).toBe(aSet.size); // no duplicate neighbours
+    }
+    expect(checked).toBeGreaterThan(0);
+  });
+
+  it('spot-checks known real bordering pairs', () => {
+    // Brazil ↔ Peru (Acre borders Loreto) — a real, unambiguous land border.
+    expect(adjacency['acre-ekzw']?.includes('loreto-ipkx')).toBe(true);
+    expect(adjacency['loreto-ipkx']?.includes('acre-ekzw')).toBe(true);
+    // France ↔ Germany (Moselle borders Baden-Württemberg) — another real border.
+    expect(adjacency['moselle-rygy']?.includes('baden-wurttemberg-oyje')).toBe(true);
+    expect(adjacency['baden-wurttemberg-oyje']?.includes('moselle-rygy')).toBe(true);
+  });
+});