perf(rpn): cache Cool metric pointer to avoid repeated RWMutex lookups

GetCoolMetric() is called for every NewFloat/NewRat/NewNumber.
Previously acquired RWMutex read lock via GetMetricRegistry().Find()
each time. Now caches the *Metric pointer during registry initialization,
returning a bare pointer after first call with no lock needed.

2 files changed, 6 insertions, 0 deletions

diff --git a/internal/rpn/metric_registry.go b/internal/rpn/metric_registry.go
index a214737..de6b1a4 100644
--- a/internal/rpn/metric_registry.go
+++ b/internal/rpn/metric_registry.go
@@ -20,12 +20,14 @@ type MetricRegistry struct {
 // Global registry instance.
 var defaultRegistry *MetricRegistry
 var registryOnce sync.Once
+var cachedCoolMetric *Metric // cached Cool metric pointer (set after init)
 
 // GetMetricRegistry returns the global metric registry, initialized with built-in metrics.
 func GetMetricRegistry() *MetricRegistry {
 	registryOnce.Do(func() {
 		defaultRegistry = NewMetricRegistry()
 		registerBuiltInMetrics(defaultRegistry)
+		cachedCoolMetric, _ = defaultRegistry.Find("Cool")
 	})
 	return defaultRegistry
 }
diff --git a/internal/rpn/number.go b/internal/rpn/number.go
index 9a7ac99..b82c4f6 100644
--- a/internal/rpn/number.go
+++ b/internal/rpn/number.go
@@ -56,7 +56,11 @@ func NewNumber(value float64, mode CalculationMode, metric ...*Metric) Number {
 }
 
 // GetCoolMetric returns the universal default metric.
+// Returns a cached pointer after first call — no lock needed.
 func GetCoolMetric() *Metric {
+	if cachedCoolMetric != nil {
+		return cachedCoolMetric
+	}
 	m, _ := GetMetricRegistry().Find("Cool")
 	return m
 }