fix: Enable runtime calls to mp_units::inverse() on Clang 18-20

Problem: Clang 18-20 strict immediate function enforcement prevents runtime
calls to mp_units::inverse() due to internal consteval operators.

Solution: Modified MP_UNITS_CONSTEVAL to use constexpr for Clang <21,
consteval for Clang >=21. Preserves compile-time optimization while
enabling runtime usage.

Testing: Added comprehensive runtime tests for inverse() function covering
time-to-frequency conversion, runtime parameters, and unit conversion.

🤖 Generated with [Claude Code](https://claude.ai/code)

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

Author: rothmichaels

src/core/include/mp-units/bits/hacks.h

@@ -122,7 +122,7 @@ MP_UNITS_DIAGNOSTIC_POP
 
 #endif
 
-#if defined MP_UNITS_COMP_CLANG && MP_UNITS_COMP_CLANG < 17
+#if defined MP_UNITS_COMP_CLANG && MP_UNITS_COMP_CLANG < 21
 
 #define MP_UNITS_CONSTEVAL constexpr
 

test/runtime/math_test.cpp

@@ -589,4 +589,31 @@ TEST_CASE("math operations", "[math]")
       REQUIRE_THAT(atan2(1. * isq::length[km], 1000. * isq::length[m]), AlmostEquals(45. * angle[deg]));
     }
   }
+
+  SECTION("inverse functions")
+  {
+    SECTION("inverse of time quantity returns frequency")
+    {
+      auto period = 2.0 * isq::time[s];
+      auto frequency = inverse<si::hertz>(period);
+      REQUIRE(frequency == 0.5 * isq::frequency[Hz]);
+    }
+
+    SECTION("inverse works with runtime values")
+    {
+      // Test the specific case that fails with consteval
+      double runtime_value = 3.0;
+      auto period = runtime_value * isq::time[s];
+      auto frequency = inverse<si::hertz>(period);
+      auto expected = (1.0/3.0) * isq::frequency[Hz];
+      REQUIRE_THAT(frequency, AlmostEquals(expected));
+    }
+
+    SECTION("inverse with different input units")
+    {
+      auto period_ms = 500.0 * isq::time[ms];
+      auto frequency = inverse<si::hertz>(period_ms);
+      REQUIRE(frequency == 2.0 * isq::frequency[Hz]);
+    }
+  }
 }