feat(#52): scenario generator LLM steps + deterministic stub

- server/scenario.ts: buildGeneratePrompt / buildRegeneratePrompt, defensive
  parseGenerate / parseRegenerate (fence-strip + balanced-object + validate),
  generateDraft (draft world + advisor checkpoints) and finalizeWorld (final
  world constrained to approved checkpoints -> playable GameState), all over
  the injectable LlmClient with a nudge re-send. SCENARIO_* markers for the
  stub.
- server/stubScenario.ts: deterministic scenario stub transport (fixed 3-nation
  world + checkpoint list; finalize refines stability per approved checkpoint)
  + makeScenarioStubLlmClient. Distinct from and never touching turn stub.ts.
- server/regions.ts: validCountryCodes() accessor (ISO_A3 parents) for draft
  territory validation.
- shared/scenario.ts: worldDraftToGameState now guards a dangling playerNationId
  (falls back to the first nation) so a finalize that drops it stays playable.
- server/scenario.test.ts: 16 tests (prompt builders, defensive parsing,
  generateDraft/finalizeWorld over a fake client + the stub).

Fixes #52

shared/scenario.ts

@@ -385,6 +385,17 @@ export function worldDraftToGameState(
     };
   }
 
+  // Guard: a finalize step (or a bad model answer) might drop the nation the
+  // player chose to lead. Rather than produce a game with a dangling
+  // playerNationId, fall back to the first nation (deterministic insertion
+  // order) so the resulting game is always playable.
+  const effectivePlayer =
+    nations[playerNationId] !== undefined
+      ? playerNationId
+      : draft.nations.length > 0
+        ? draft.nations[0].id
+        : '';
+
   // Seed the event feed from the draft's optional backstory (turn 1, startDate).
   const events: EventEntry[] = (draft.events ?? []).map((e) =>
     makeEvent(e as unknown as TurnEventSpec, 1, startDate),
@@ -394,7 +405,7 @@ export function worldDraftToGameState(
     id,
     presetId: 'dynamic',
     presetTitle: title,
-    playerNationId,
+    playerNationId: effectivePlayer,
     turn: 1,
     date: startDate,
     nations,