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/regions.test.ts

@@ -118,6 +118,8 @@ describe('regions (region-key loader, v0.19 #16 P1)', () => {
       embargoes: [],
       // (v0.36.0, #59) unused here (allTerritoryKeys is ownership-only).
       industry: [],
+      // (v0.43.0, #65) no sea-zone control on a fresh state.
+      seaZones: [],
     };
     // 16 GBR sub-regions + 42 FRA sub-regions, no overlap between the two.
     expect(allTerritoryKeys(state).size).toBe(58);