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/engine.ts

@@ -24,6 +24,7 @@ import {
 } from '../shared/protocol';
 import { DEFAULT_MAX_TOKENS, type LlmClient } from './llm';
 import { ADVISOR_MARKER } from './stub';
+import { regionNamesFor } from './regions';
 
 const VALID_EVENT_TYPES = new Set<string>(EVENT_TYPES);
 const VALID_RELATIONS = new Set<string>(RELATION_STATUSES);
@@ -53,11 +54,12 @@ export function buildSystemPrompt(preset: Preset): string {
     '  ],',
     '  "stateChanges": [{"nation":"<nationId>","stat":"<military|economy|stability|population>","op":"<set|add>","value":<0-100>}],',
     '  "relations": [{"a":"<nationId>","b":"<nationId>","set":"<ally|neutral|rival|hostile|war>"}],',
-    '  "territoryChanges": [{"nation":"<nationId>","gain":["<ISO_A3>"],"lose":["<ISO_A3>"]}]',
+    '  "territoryChanges": [{"nation":"<nationId>","gain":["<regionId>"],"lose":["<regionId>"]}]',
     '}',
     '',
     'HARD CONSTRAINTS',
-    '- Use only nation ids and ISO_A3 codes that exist in the provided state.',
+    '- Use only nation ids and sub-region ids (e.g. `FR-57`) that exist in the provided state.',
+    '- A region id (e.g. `FR-57`) identifies a single sub-region — the `regions` table below lists each id with its name and country; a nation\'s border can change one region at a time.',
     '- Prefer 1-4 events per turn; keep each text 1-3 sentences.',
     '- Keep your internal reasoning brief (a few sentences) and go straight to the JSON result; do not over-deliberate or re-derive facts already present in the state.',
     '- Keep every stat value between 0 and 100.',
@@ -106,6 +108,15 @@ export function buildUserPrompt(state: GameState, items: RoundItem[]): string {
         outcome: e.outcome,
       })),
     },
+    // A name lookup for every region id referenced anywhere in the state, so the
+    // model can map an id it is considering (e.g. `FR-57`) to a place it
+    // recognizes. This is a top-level reference table keyed by sub-region id;
+    // each nation's own `territory` still carries raw ids (via nationForPrompt,
+    // unchanged) so the deterministic stub can parse them. When the region geo
+    // is not loaded the table is empty and the model falls back to the raw ids.
+    regions: regionNamesFor(
+      Object.values(state.nations).flatMap((n) => n.territory),
+    ),
     playerActions,
     pending: items,
     roundInstruction,