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/near-future.json

@@ -0,0 +1,17 @@
+{
+  "id": "near-future",
+  "name": "Near Future",
+  "year": 2035,
+  "eraDescription": "A near-future world: autonomous fleets, orbital assets, and climate-hamstrung coastlines. Old alliances strain under new technologies and old grievances as the great powers reach for the next horizon.",
+  "suggestedPowers": [
+    { "name": "North American Union", "region": "USA", "note": "The industrial-technological core of the West." },
+    { "name": "Chinese Federation", "region": "CHN", "note": "The integrated superpower of the Pacific." },
+    { "name": "European Confederation", "region": "FRA", "note": "The united European bloc." },
+    { "name": "Russian Union", "region": "RUS", "note": "The resource frontier state." }
+  ],
+  "tensions": [
+    "Autonomous weapons and the new arms race",
+    "Climate migration and resource wars",
+    "The contest for space and orbital dominance"
+  ]
+}