feat(#52): era templates + shared scenario types/transforms

- presets/templates/: 3 light era templates (ww2/present/near-future) + index.json
  manifest (id, name, year, eraDescription, suggestedPowers, tensions).
- server/templates.ts: createTemplateStore (mirror of createPresetStore).
- shared/scenario.ts: dependency-free contract for the dynamic scenario path —
  SuggestedPower / EraTemplate / TemplateMeta / WorldNation / ScenarioEvent /
  WorldDraft / Checkpoint types, relationFromScore, validateWorldDraft (pure,
  defensive, never throws), worldDraftToGameState (region-key territory via the
  injected expand resolver, regionStats seeded, clean turn-1 state).
- tests: server/templates.test.ts (manifest order + file shape),
  server/scenario-shared.test.ts (15 cases: validation/clamp/dedup/null +
  transform).

Fixes #52

server/templates.ts

@@ -0,0 +1,28 @@
+// Load era templates for the "Create scenario" path from disk (v0.31.0, #52).
+// Mirrors the preset store (server/presets.ts): each template is a JSON file
+// (presets/templates/<id>.json) and index.json is the lightweight manifest used
+// to render the picker cards. The store is created with the resolved templates
+// directory so it works from repo root (dev) and /app (Docker) alike.
+import { readFileSync } from 'node:fs';
+import type { EraTemplate, TemplateMeta } from '../shared/scenario';
+
+export interface TemplateStore {
+  listMeta(): TemplateMeta[];
+  get(id: string): EraTemplate | null;
+}
+
+export function createTemplateStore(dir: string): TemplateStore {
+  const manifest = JSON.parse(readFileSync(`${dir}/index.json`, 'utf8')) as TemplateMeta[];
+  return {
+    listMeta() {
+      return manifest;
+    },
+    get(id: string) {
+      try {
+        return JSON.parse(readFileSync(`${dir}/${id}.json`, 'utf8')) as EraTemplate;
+      } catch {
+        return null;
+      }
+    },
+  };
+}