feat: sub-region-level territory — ownership, LLM and map at region granularity (v0.19.0)

Move territory from whole-country (ISO_A3) to individual sub-region id
granularity (PR 3 of #16):

- new server/regions.ts (turf-free): loadRegions (boot), expandTerritory
  (game creation), migrateTerritoryToRegions (legacy games on db.load),
  regionNamesFor / regionLabel (LLM prompt), graceful ISO_A3 fallback
- game creation expands each preset nation's ISO_A3 list to its sub-region
  ids; pre-v0.19 saved games migrate transparently on read (idempotent)
- LLM contract: territoryChanges gain/lose sub-region ids (one region at a
  time; gain moves out of the prior owner; lost-without-gainer = unowned);
  user prompt carries a top-level regions reference table
  ({id: 'name (parent)'} over the state's territory ids)
- WorldMap: choropleth fill per sub-region (countries can split between
  nations), ownership via properties.regionId through the shared
  mapFeatureOwner, flash lights exactly the changed features
  (featuresByRegionIds)
- deterministic stub: one region-level move per non-empty round (player
  annexes the lexicographically-first region of the first other nation)
- tests: new server/regions.test.ts (real geo + fallback + seams);
  map/engine tests re-pinned to region ids; scripts/regions.test.ts stays green

Measured full wwii-1939 user prompt (411 regions): ~24.7k chars / ~6.2k
tokens — far under the 80k frontier limit, so the full regions table is
always sent (no frontier-limited mode needed).

Part of #16

server/db.ts

@@ -3,6 +3,7 @@ import { mkdirSync } from 'node:fs';
 import path from 'node:path';
 import Database from 'better-sqlite3';
 import type { GameState } from '../shared/types';
+import { migrateTerritoryToRegions } from './regions';
 
 export interface Db {
   save(state: GameState): void;
@@ -39,7 +40,11 @@ export function openDb(filePath: string): Db {
       const row = byId.get(id) as { data: string } | undefined;
       if (!row) return null;
       try {
-        return JSON.parse(row.data) as GameState;
+        // v0.19: migrate any pre-v0.19 (whole-country ISO_A3) territory to
+        // sub-region ids on read. A no-op when the region geo is not loaded,
+        // and idempotent once a state has been migrated (the next save then
+        // persists the region-level territory).
+        return migrateTerritoryToRegions(JSON.parse(row.data) as GameState);
       } catch {
         return null;
       }