feat(#52): wire /api/scenarios routes + dynamic-game jump preset

- GET /api/scenarios/templates: the 3 era templates (templateStore from
  presets/templates/).
- POST /api/scenarios/draft: generateDraft (template + description + year) ->
  { world, checkpoints } (502 on transport failure / unusable world).
- POST /api/scenarios/commit: finalizeWorld (constrained to approved
  checkpoints, optionally based on the re-sent draft) -> saved GameState,
  returns { id } (a normal game at #/game/<id>).
- scenarioLlm: the deterministic scenario stub under LLM_STUB=1 (distinct
  from the untouched turn stub), the live client otherwise.
- jump route: synthesize a minimal Preset for dynamic games (presetId
  'dynamic', no static file) so their turns run; genuinely-missing static
  presets still 404.

Fixes #52

server/index.ts

@@ -17,13 +17,17 @@ import type {
   RejectNationOpRequest,
   RemoveQueueItemRequest,
 } from '../shared/protocol';
-import type { GameState } from '../shared/types';
+import type { GameState, Preset } from '../shared/types';
+import type { WorldDraft } from '../shared/scenario';
 import { createLlmClient } from './llm';
 import { makeStubLlmClient } from './stub';
+import { makeScenarioStubLlmClient } from './stubScenario';
+import { finalizeWorld, generateDraft } from './scenario';
 import { askAdvisor, confirmNationOp, queueRoundItem, rejectNationOp, removeRoundItem, resolveRegionKey, runTurn } from './engine';
 import { buildRegionMap } from './map';
-import { allTerritoryKeys, centroidOf, expandCountriesToKeys, geoOf, hasAdjacency, hasCentroids, hasGeo, loadRegions, neighboursOf, referenceForKeys, regionName, validKeys } from './regions';
+import { allTerritoryKeys, centroidOf, expandCountriesToKeys, geoOf, hasAdjacency, hasCentroids, hasGeo, loadRegions, neighboursOf, referenceForKeys, regionName, validCountryCodes, validKeys } from './regions';
 import { createPresetStore } from './presets';
+import { createTemplateStore } from './templates';
 import { openDb, type Db } from './db';
 
 const here = fileURLToPath(new URL('.', import.meta.url));
@@ -52,6 +56,8 @@ const dataDir = process.env.DATA_DIR ?? path.join(root, 'data');
 // the e2e config, so honor it as a fallback to keep all three in agreement.
 const PORT = parseInt(process.env.PORT ?? process.env.HISTORY_API_PORT ?? '4321', 10);
 const presetStore = createPresetStore(presetsDir);
+// (v0.31, #52) Era templates for the "Create scenario" path (presets/templates/).
+const templateStore = createTemplateStore(path.join(root, 'presets', 'templates'));
 // Load the region-key manifest + geo once at boot (v0.19 #16 P1). A missing file
 // just means region expansion/reference degrade to the raw keys (server still boots).
 loadRegions(root);
@@ -59,6 +65,10 @@ const db: Db = openDb(path.join(dataDir, 'history.db'));
 // LLM_STUB=1 → deterministic stub (tests/e2e/dev without a live model).
 const useStub = process.env.LLM_STUB === '1';
 const llm = useStub ? makeStubLlmClient() : createLlmClient();
+// (v0.31, #52) The scenario-generation LLM client: the deterministic scenario
+// stub under LLM_STUB=1 (a distinct transport from the turn stub, which stays
+// untouched), and the live client otherwise.
+const scenarioLlm = useStub ? makeScenarioStubLlmClient() : llm;
 
 const app = express();
 app.use(express.json());
@@ -95,6 +105,34 @@ function broadcast(id: string, event: string, data: unknown): void {
 
 // --- Routes ---
 
+/**
+ * (v0.31, #52) Synthesize the minimal `Preset` the turn engine needs for a
+ * dynamically generated game (`presetId: 'dynamic'` — there is no static preset
+ * file on disk). `runTurn` only consumes `title` / `startDate` /
+ * `worldBackground` / `simulationRules` (never `preset.nations`), so a generic
+ * alternate-history background plus the standard rules is sufficient; `nations`
+ * is carried from the saved world so the type is complete. `startDate` uses the
+ * game's current date (GameState stores no separate start date) — it only feeds
+ * the static "WORLD: <title> — <date>" header of the system prompt.
+ */
+function dynamicPresetFromGame(game: GameState): Preset {
+  return {
+    id: game.presetId,
+    title: game.presetTitle,
+    startDate: game.date,
+    summary: game.presetTitle,
+    worldBackground: `${game.presetTitle} is a dynamically generated alternate-history world. Keep every nation's stats, borders and relations internally consistent as the world evolves.`,
+    simulationRules: [
+      "Advance the shared world for ALL nations, not only the player's.",
+      'Keep every stat value between 0 and 100.',
+      'Only change borders when war or a decisive political shift justifies it.',
+      'Prefer 1-4 events per turn; keep each narrative to 1-3 sentences.',
+      'Use only nation ids and region keys that exist in the provided state.',
+    ],
+    nations: Object.values(game.nations),
+  };
+}
+
 app.get('/api/health', (_req, res) => {
   res.json({ ok: true, stub: useStub, port: PORT });
 });
@@ -278,8 +316,14 @@ app.post('/api/games/:id/jump', async (req, res) => {
   const id = req.params.id;
   const game = db.load(id);
   if (!game) return res.status(404).json({ error: 'game not found' });
-  const preset = presetStore.get(game.presetId);
-  if (!preset) return res.status(404).json({ error: 'preset_missing' });
+  let preset = presetStore.get(game.presetId);
+  if (!preset) {
+    // (v0.31, #52) A dynamically generated game has no static preset file:
+    // synthesize one from the saved world so the turn engine can run. A
+    // genuinely-missing static preset still 404s.
+    if (game.presetId !== 'dynamic') return res.status(404).json({ error: 'preset_missing' });
+    preset = dynamicPresetFromGame(game);
+  }
   try {
     const { state, result } = await runTurn({
       llm,
@@ -347,6 +391,86 @@ app.post('/api/games/:id/advisor', async (req, res) => {
   }
 });
 
+// --- Dynamic scenario generation (v0.31, #52) ---
+//
+// The additive "Create scenario" path. Step 1 (POST /draft) designs a draft
+// world + an advisor checkpoint list; the client shows both, the player
+// approves/removes checkpoints and picks a nation; step 2 (POST /commit)
+// finalizes the world (constrained to the approved checkpoints) into a saved,
+// playable game. Both steps ride the scenario LLM client (the deterministic
+// scenario stub under LLM_STUB=1, the live model otherwise). All existing
+// routes are untouched.
+
+app.get('/api/scenarios/templates', (_req, res) => {
+  res.json({ templates: templateStore.listMeta() });
+});
+
+app.post('/api/scenarios/draft', async (req, res) => {
+  const body = (req.body ?? {}) as { templateId?: string; description?: string; year?: number };
+  const template = templateStore.get(body.templateId ?? '');
+  if (!template) return res.status(400).json({ error: 'unknown template' });
+  const description = typeof body.description === 'string' ? body.description.trim() : '';
+  const year = Number.isFinite(body.year as number) ? (body.year as number) : template.year;
+  const codes = validCountryCodes();
+  try {
+    const { world, checkpoints } = await generateDraft({
+      llm: scenarioLlm,
+      template,
+      description,
+      year,
+      // No region data → no code filtering (the draft keeps whatever codes the
+      // model emitted; expansion degrades to the raw keys, like presets do).
+      validCountryCodes: codes.size > 0 ? codes : undefined,
+    });
+    if (!world) return res.status(502).json({ error: 'generate: the model returned no usable world' });
+    res.json({ world, checkpoints });
+  } catch (e) {
+    const msg = e instanceof Error ? e.message : String(e);
+    res.status(502).json({ error: `generate failed: ${msg}` });
+  }
+});
+
+app.post('/api/scenarios/commit', async (req, res) => {
+  const body = (req.body ?? {}) as {
+    templateId?: string;
+    description?: string;
+    year?: number;
+    draft?: WorldDraft;
+    approvedCheckpoints?: string[];
+    playerNationId?: string;
+  };
+  const template = templateStore.get(body.templateId ?? '');
+  if (!template) return res.status(400).json({ error: 'unknown template' });
+  const description = typeof body.description === 'string' ? body.description.trim() : '';
+  const year = Number.isFinite(body.year as number) ? (body.year as number) : template.year;
+  const approvedCheckpoints = Array.isArray(body.approvedCheckpoints)
+    ? body.approvedCheckpoints.filter((c): c is string => typeof c === 'string' && c.trim() !== '')
+    : [];
+  const playerNationId = typeof body.playerNationId === 'string' ? body.playerNationId : '';
+  const codes = validCountryCodes();
+  try {
+    const state = await finalizeWorld({
+      llm: scenarioLlm,
+      template,
+      description,
+      year,
+      approvedCheckpoints,
+      draft: body.draft,
+      playerNationId,
+      validCountryCodes: codes.size > 0 ? codes : undefined,
+      expand: (cs) => expandCountriesToKeys(cs),
+      id: randomUUID(),
+      title: template.name,
+      startDate: `${year}-01-01`,
+    });
+    db.save(state);
+    res.json({ id: state.id });
+  } catch (e) {
+    const msg = e instanceof Error ? e.message : String(e);
+    res.status(502).json({ error: `commit failed: ${msg}` });
+  }
+});
+
 // --- SSE stream ---
 app.get('/sse/:id', (req, res) => {
   const id = req.params.id;
@@ -384,4 +508,4 @@ app.listen(PORT, () => {
 });
 
 // Exported for tests (createApp is implicit via app; here we just expose handles).
-export { app, db, llm };
+export { app, db, llm, scenarioLlm, templateStore };