Add derived constants validation tests for PhysicalConstants

This update introduces comprehensive tests in PhysicalConstantsTests.cs to validate derived constants against calculated values. The tests cover area conversions, time relationships, and temperature conversions, ensuring accuracy and consistency across various derived unit relationships. This enhancement improves the robustness of the physical constants validation in the Semantics library, aligning with the recent refactor to centralize unit conversions in PhysicalConstants.

Author: matt-edmondson

.cursor/rules/derived-cursor-rules.mdc

@@ -220,4 +220,5 @@ When adding XML documentation comments:
     *   Metric prefixes (handled by `MetricMagnitudes` class): `0.001`, `1000.0`, etc.
     *   Mathematical constants (like `Math.PI`): Already using C# standard library
     *   Derived unit relationships (like time conversions): `60.0` (seconds per minute), `3600.0` (seconds per hour)
-    *   Imperial unit conversions not in PhysicalConstants: Many specific imperial conversions that aren't fundamental physical constants
+    *   Imperial unit conversions not in PhysicalConstants: Many specific imperial conversions that aren't fundamental physical constants
+*   **Testing Derived Constants:** Add additional tests for physical constants to validate stored values against calculated values from fundamental constants.