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

presets/templates/index.json

@@ -0,0 +1,20 @@
+[
+  {
+    "id": "ww2",
+    "name": "World War II",
+    "year": 1939,
+    "eraDescription": "Europe teeters on the brink of total war. Fascist powers press their advantage across the continent while the democracies marshal their strength."
+  },
+  {
+    "id": "present",
+    "name": "Present Day",
+    "year": 2026,
+    "eraDescription": "A multipolar world of blocs, sanctions, and tech supremacy. Climate strain, AI, and great-power rivalry shape every decision."
+  },
+  {
+    "id": "near-future",
+    "name": "Near Future",
+    "year": 2035,
+    "eraDescription": "Autonomous fleets, orbital assets, and climate-hamstrung coastlines. Old alliances strain under new technologies and old grievances."
+  }
+]