Split °F temperature assertion to satisfy type-checker

The single-expression `1°F` assertion in testTemperature() exceeds the
Swift type-checker's per-expression time budget on the macOS CI toolchain
once Quantity becomes a RawRepresentable struct. Break it into typed
sub-expressions; arithmetic and assertion are unchanged.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: rstacpoole

Tests/UnitsTests/DefinitionTests.swift

@@ -263,7 +263,16 @@ class DefinitionTests: XCTestCase {
         // Base unit: kelvin
         XCTAssertEqual(Measurement("1K"), 1.measured(in: .kelvin))
         try XCTAssertEqual(Measurement("1°C"), (273.15.measured(in: .kelvin) + 1.measured(in: .kelvin)).convert(to: .celsius))
-        try XCTAssertEqual(XCTUnwrap(Measurement("1°F")), try ((273.15 - (32 * 5.0 / 9.0)).measured(in: .kelvin) + (5.0 / 9.0).measured(in: .kelvin)).convert(to: .fahrenheit), accuracy: 0.0001)
+        // Broken into sub-expressions with explicit types: as a single expression
+        // this overflows the type-checker's per-expression budget on some toolchains.
+        let fahrenheitOffset: Double = 273.15 - (32 * 5.0 / 9.0)
+        let fahrenheitScale: Double = 5.0 / 9.0
+        let oneFahrenheit = try fahrenheitOffset.measured(in: .kelvin) + fahrenheitScale.measured(in: .kelvin)
+        try XCTAssertEqual(
+            XCTUnwrap(Measurement("1°F")),
+            oneFahrenheit.convert(to: .fahrenheit),
+            accuracy: 0.0001
+        )
         try XCTAssertEqual(Measurement("1°R"), (5.0 / 9.0).measured(in: .kelvin).convert(to: .rankine))
     }