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

package.json

@@ -1,6 +1,6 @@
 {
   "name": "history",
-  "version": "0.18.0",
+  "version": "0.19.0",
   "private": true,
   "type": "module",
   "description": "AI-driven alternate-history simulator — the LLM is the world engine (Pax Historia style).",