feat: Adds furnace heating interpolations.

Tests/ManualSTests/ManualSTests.swift

@@ -159,6 +159,51 @@ struct ManualSTests {
     }
   }
 
+  @Test
+  func furnaceInterpolation() async throws {
+    try await withDependencies {
+      $0.manualS = .liveValue
+    } operation: {
+      @Dependency(\.manualS) var manualS
+
+      let response = try await manualS.furnaceInterpolation(
+        .init(
+          elevation: nil,
+          winterDesignTemperature: 5,
+          heatLoss: 49667,
+          inputRating: 60000,
+          afue: 96
+        )
+      )
+
+      #expect(response.failed == false)
+      #expect(response.capacityAsPercentOfLoad == 116)
+      #expect(response.outputCapacity == 57600)
+    }
+  }
+
+  @Test
+  func furnaceInterpolation_with_oversizedFurnace() async throws {
+    try await withDependencies {
+      $0.manualS = .liveValue
+    } operation: {
+      @Dependency(\.manualS) var manualS
+
+      let response = try await manualS.furnaceInterpolation(
+        .init(
+          elevation: nil,
+          winterDesignTemperature: 5,
+          heatLoss: 49667,
+          inputRating: 160_000,
+          afue: 96
+        )
+      )
+
+      #expect(response.failed)
+      #expect(response.failures == ["Oversizing failure."])
+    }
+  }
+
   @Test
   func balancePoint() async throws {
     try await withDependencies {