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.

package.json

@@ -15,6 +15,7 @@
     "smooth:regions": "node scripts/smooth-regions.mjs",
     "fill:regions": "node scripts/fill-gaps.mjs",
     "build:region-keys": "node scripts/build-region-keys.mjs",
+    "build:region-adjacency": "node scripts/build-region-adjacency.mjs",
     "typecheck": "tsc --noEmit",
     "lint": "eslint .",
     "e2e": "playwright test"