Flip ownership to region keys: shared contract + server (v0.19 P1 of #16)

The single source of truth now keys ownership by region key (one per owned
sub-region) instead of a parent-country ISO_A3:

- shared/types.ts, shared/protocol.ts: TerritoryChange / RegionMap / regionOwner
  doc flips; mapFeatureOwner now reads the feature's stamped properties.regionKey
  (never the repacked numeric feature.id, nor the sub-region id — the #6 guard).
- server/regions.ts (new, turf-free): loads region-keys.json + regions.geo.json
  once at boot; expands a nation's ISO_A3 list to its sub-regions' keys, builds
  the LLM 'regions' reference table (key -> "name (parent)"), and exposes the
  set of valid keys. Degrades gracefully when the files are missing.
- server/engine.ts: buildSystemPrompt instructs the model to use region keys;
  buildUserPrompt gains an optional regionReference table; applyTurnResult gains
  an optional validKeys set so hallucinated keys are ignored.
- server/index.ts: loads region data at boot; expands each nation's country list
  to region keys at game creation; the jump route passes the reference + valid
  keys through runTurn.

The stub is unchanged (it emits an empty, region-key-conformant territoryChanges).

server/map.ts

@@ -1,6 +1,8 @@
-// Build the choropleth payload from a GameState: every ISO_A3 region maps to its
-// owning nation plus that nation's stable color (see shared.nationColor). The
-// client merges this onto public/countries.geo.json and calls source.setData().
+// Build the choropleth payload from a GameState: every region key (see
+// public/region-keys.json) maps to its owning nation plus that nation's stable
+// color (see shared.nationColor). The client merges this onto the sub-national
+// regions.geo.json (resolving each feature by its own region key) and calls
+// source.setData(). Format-agnostic: it maps whatever keys sit in territory.
 import { nationColor, type RegionMap } from '../shared/protocol';
 import type { GameState } from '../shared/types';
 
@@ -8,8 +10,8 @@ export function buildRegionMap(state: GameState): RegionMap {
   const map: RegionMap = {};
   for (const [id, nation] of Object.entries(state.nations)) {
     const color = nationColor(id);
-    for (const code of nation.territory) {
-      map[code] = { color, owner: nation.name, ownerId: id };
+    for (const key of nation.territory) {
+      map[key] = { color, owner: nation.name, ownerId: id };
     }
   }
   return map;