feat(#61): named leaders — assassination / death / appointment with succession (v0.38.0)

Per-nation named leaders (Nation.leader) and a required TurnResult.leaders
lever ({ changes: [{ target, name, cause }] }). Each change sets the target
nation's leader and applies a deterministic stability dip by cause
(assassination -15, death -3, appointed 0) mirrored to its regions and
re-derived into its stats, plus a narration event (military for an
assassination, political for a death/appointment). An assassination/death
requires the target to already have a leader (a leaderless target is
dropped); an appointment always creates/sets one. Unknown/ambiguous targets
are dropped, never thrown.

- shared/types.ts: optional Nation.leader
- shared/protocol.ts: LeaderCause/LeaderChange/LeaderOps, LEADER_STABILITY_EFFECT,
  TurnResult.leaders (required, between nuclear and markers), leader in nationForPrompt
- server/engine.ts: validateLeaderOps, applyLeaderChange, the applyTurnResult
  leader block (after nuclear, before markers), isMeaningful, system-prompt
  contract + constraint, validateTurnResult + runTurn fallback
- server/stub.ts: leaders always { changes: [] } (e2e unaffected)
- tests: 15 engine.test.ts leader tests + 2 db.test.ts round-trips
- version 0.37.0 -> 0.38.0, CHANGELOG entry

server/db.test.ts

@@ -200,6 +200,28 @@ describe('openDb', () => {
     db.close();
   });
 
+  it('(v0.38.0 #61) a legacy row saved before the nation leader field existed loads with it absent (no leader)', () => {
+    const db = openDb(':memory:');
+    // makeState has no nation.leader field — a pre-v0.38 save. The field is
+    // optional and not re-stamped by load, so an absent leader stays absent.
+    db.save(makeState('legacy'));
+    const loaded = db.load('legacy');
+    expect(loaded).not.toBeNull();
+    expect(loaded!.nations.uk.leader).toBeUndefined();
+    db.close();
+  });
+
+  it('(v0.38.0 #61) a set nation leader round-trips through save/load', () => {
+    const db = openDb(':memory:');
+    const s = makeState('ldr');
+    s.nations.uk.leader = { name: 'Admiral Old' };
+    db.save(s);
+    const loaded = db.load('ldr');
+    expect(loaded).not.toBeNull();
+    expect(loaded!.nations.uk.leader).toEqual({ name: 'Admiral Old' });
+    db.close();
+  });
+
   it('delete removes a game by id and reports absence otherwise', () => {
     const db = openDb(':memory:');
     db.save(makeState('a'));