feat(#56): War as a first-class state — declare/end ops, mutual-defense drag-in, relation sync

Promote the bare 'war' relation to a named War entity in GameState.wars:
- War: { id, sides[], declaredTurn, declaredDate?, aims?, status, front? }
- TurnResult.wars: { declare: WarDeclareOp[], end: WarEndOp[] }
- syncWarRelations: derive 'war' from active wars, fallback to 'hostile' on end
- Mutual-defense drag-in: belligerent allies added to sides (one level)
- Bare 'war' back-compat: never downgraded, never force-created
- Stub: empty wars (e2e unaffected); ~30 new unit tests

v0.33.0

server/db.test.ts

@@ -31,6 +31,8 @@ function makeState(id: string): GameState {
     regionStats: {
       GBR: { military: 80, economy: 70, stability: 60, population: 50 },
     },
+    // (v0.33.0, #56) no wars.
+    wars: [],
   };
 }