feat: #65 sea control — 11th ability (v0.43.0)

Adds the sea ability: a fixed set of 12 named sea zones with sole-control
semantics, contest-by-higher-military, and a deterministic per-turn
+1 military/+1 economy bonus for each claimed zone. Durable (owns
GameState.seaZones), additive migration, stub-inert (e2e unaffected).

Fixes #65

Design note: Gitea #65 sketched a free-form 'naval' layer; the implemented
approved design is this richer 'sea' layer with a fixed zone set.

server/db.test.ts

@@ -38,6 +38,8 @@ function makeState(id: string): GameState {
     embargoes: [],
     // (v0.36.0, #59) no industry projects.
     industry: [],
+    // (v0.43.0, #65) no sea-zone control on a fresh state.
+    seaZones: [],
   };
 }
 
@@ -244,6 +246,17 @@ describe('openDb', () => {
     db.close();
   });
 
+  it('(v0.43.0 #65) a legacy row saved before the seaZones field existed loads with it defaulted to []', () => {
+    const db = openDb(':memory:');
+    const legacy = makeState('legacy');
+    delete (legacy as { seaZones?: unknown[] }).seaZones; // a pre-v0.43 save (field absent)
+    db.save(legacy);
+    const loaded = db.load('legacy');
+    expect(loaded).not.toBeNull();
+    expect(loaded!.seaZones).toEqual([]);
+    db.close();
+  });
+
   it('delete removes a game by id and reports absence otherwise', () => {
     const db = openDb(':memory:');
     db.save(makeState('a'));