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/abilities/migration.test.ts

@@ -82,6 +82,8 @@ function mkState(
     trades: [],
     embargoes: [],
     industry: [],
+    // (v0.43.0, #65) no sea-zone control on a fresh state.
+    seaZones: [],
   };
 }