feat(engine): add zone bounds to VisibleState for bot awareness

Per plan §3.7, the shrinking zone is a forcing function that should
force combat engagement. Previously, bots could not see the current
zone state (center, radius, active) and would move away from the zone
center, dying without understanding why.

Changes:
- Add Zone field to VisibleState (types.go)
- Populate zone bounds in GetVisibleState() when zone enabled (game.go)

This allows HTTP and local bots to see the zone and react strategically
(e.g., move toward center to avoid zone death while engaging enemies).

Test: zone bounds now appear in VisibleState JSON with correct values.

Closes: bf-tfyy

engine/game.go

@@ -326,6 +326,15 @@ func (gs *GameState) GetVisibleState(playerID int) *VisibleState {
 		}
 	}
 
+	// Include zone bounds if enabled
+	if gs.Config.ZoneEnabled {
+		vs.Zone = &ZoneBounds{
+			Center: gs.ZoneCenter,
+			Radius: gs.ZoneRadius,
+			Active: gs.ZoneActive,
+		}
+	}
+
 	return vs
 }
 

engine/types.go

@@ -291,6 +291,7 @@ type VisibleState struct {
 	Cores  []VisibleCore `json:"cores"`
 	Walls  []Position    `json:"walls"`
 	Dead   []VisibleBot  `json:"dead"`
+	Zone   *ZoneBounds   `json:"zone,omitempty"` // Current zone state (nil if zone disabled)
 }
 
 // VisibleBot represents a bot visible to a player.