Add region-key manifest (build:region-keys) + public/region-keys.json (v0.19 P1 of #16)

Deterministic <name-slug>-<4char> key per sub-region: name via NFKD strip +
fold table (ə ø ð đ œ ł) + a-z0-9 collapse; 4-char from FNV-1a of the stable
feature id, salted on within-slug collision so all 1,796 keys are unique.
Builtins only (no @turf). Idempotent (byte-identical on re-run).

package.json

@@ -14,6 +14,7 @@
     "cleanup:regions": "node scripts/cleanup-regions.mjs",
     "smooth:regions": "node scripts/smooth-regions.mjs",
     "fill:regions": "node scripts/fill-gaps.mjs",
+    "build:region-keys": "node scripts/build-region-keys.mjs",
     "typecheck": "tsc --noEmit",
     "lint": "eslint .",
     "e2e": "playwright test"