feat: Finishes economic balance point.

Sources/Routes/Models/HeatingBalancePoint.swift

@@ -12,8 +12,29 @@ public enum HeatingBalancePoint {
   }
 
   public enum Request: Codable, Equatable, Sendable {
+    case economic(Economic)
     case thermal(Thermal)
 
+    public struct Economic: Codable, Equatable, Sendable {
+
+      public let fuelType: FuelType
+      public let fuelCostPerUnit: Double
+      public let fuelAFUE: Double
+      public let costPerKW: Double
+
+      public init(
+        fuelType: HeatingBalancePoint.FuelType,
+        fuelCostPerUnit: Double,
+        fuelAFUE: Double,
+        costPerKW: Double
+      ) {
+        self.fuelType = fuelType
+        self.fuelCostPerUnit = fuelCostPerUnit
+        self.fuelAFUE = fuelAFUE
+        self.costPerKW = costPerKW
+      }
+    }
+
     public struct Thermal: Codable, Equatable, Sendable {
 
       public let systemSize: Double
@@ -42,8 +63,32 @@ public enum HeatingBalancePoint {
   }
 
   public enum Response: Codable, Equatable, Sendable {
+    case economic(Economic)
     case thermal(Thermal)
 
+    public struct Economic: Codable, Equatable, Sendable {
+
+      public let balancePointTemperature: Double
+      public let fuelCostPerMMBTU: Double
+      public let electricCostPerMMBTU: Double
+      public let copAtBalancePoint: Double
+      public let electricFuelRatio: Double
+
+      public init(
+        balancePointTemperature: Double,
+        fuelCostPerMMBTU: Double,
+        electricCostPerMMBTU: Double,
+        copAtBalancePoint: Double,
+        electricFuelRatio: Double
+      ) {
+        self.balancePointTemperature = balancePointTemperature
+        self.fuelCostPerMMBTU = fuelCostPerMMBTU
+        self.electricCostPerMMBTU = electricCostPerMMBTU
+        self.copAtBalancePoint = copAtBalancePoint
+        self.electricFuelRatio = electricFuelRatio
+      }
+    }
+
     public struct Thermal: Codable, Equatable, Sendable {
 
       public let capacityAt47: Double
@@ -91,6 +136,26 @@ public enum HeatingBalancePoint {
       }
     }
   }
+
+  public enum FuelType: String, CaseIterable, Codable, Equatable, Sendable {
+    case naturalGas
+    case propane
+    case oil
+
+    public var label: String {
+      switch self {
+      case .propane, .oil: return "\(rawValue.capitalized)"
+      case .naturalGas: return "Natural Gas"
+      }
+    }
+
+    public var units: String {
+      switch self {
+      case .propane, .oil: return "gallons"
+      case .naturalGas: return "therm"
+      }
+    }
+  }
 }
 
 #if DEBUG
@@ -99,7 +164,13 @@ public enum HeatingBalancePoint {
     static func mock(mode: HeatingBalancePoint.Mode) -> Self {
       switch mode {
       case .economic:
-        fatalError()
+        return .economic(.init(
+          balancePointTemperature: -10.8,
+          fuelCostPerMMBTU: 27.6,
+          electricCostPerMMBTU: 38.1,
+          copAtBalancePoint: 2.24,
+          electricFuelRatio: 1.38
+        ))
       case .thermal:
         return .thermal(.init(
           capacityAt47: 24600,