fix: enable runtime usage of inverse function by using compile-time unit extraction

Replace runtime unit access (q.unit) with compile-time type extraction
(get_unit(R)) in the inverse function implementation. This prevents the
function from becoming an immediate function when consteval unit operators
are used, allowing inverse to work with runtime variables.

Key changes:
- math.h: Use get_unit(R) instead of q.unit in both code paths
- test: Add comprehensive runtime inverse tests

This surgical fix preserves maximum compile-time optimization while
enabling DSP applications that need runtime inverse calculations.

Fixes compilation error with Clang:
"call to immediate function 'inverse' is not a constant expression"

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

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

Author: rothmichaels

src/core/include/mp-units/math.h

@@ -414,12 +414,12 @@ template<Unit auto To, auto R, typename Rep>
 {
 #if MP_UNITS_API_NATURAL_UNITS
   if constexpr (!MP_UNITS_ASSOCIATED_UNIT<MP_UNITS_REMOVE_CONST(decltype(To))>)
-    return (representation_values<Rep>::one() * one).force_in(To * q.unit) / q;
+    return (representation_values<Rep>::one() * one).force_in(To * get_unit(R)) / q;
   else
 #endif
   {
     constexpr QuantitySpec auto qs = get_quantity_spec(To) * quantity<R, Rep>::quantity_spec;
-    return qs(representation_values<Rep>::one() * one).force_in(To * q.unit) / q;
+    return qs(representation_values<Rep>::one() * one).force_in(To * get_unit(R)) / q;
   }
 }
 

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]);
+    }
+  }
 }