refactor: split NewNumber into two constructors (NewNumber, NewNumberWithMetric)

Eliminate confusing variadic *Metric parameter from NewNumber().
Callers either pass nothing (defaults to Cool) or one metric —
variadic implied zero-or-many, causing confusion.

New API:
  NewNumber(value, mode)           — metric defaults to Cool
  NewNumberWithMetric(value, mode, metric) — explicit metric

All callers updated to use the appropriate constructor.
No behavioral changes; same defaults, clearer API.

7 files changed, 38 insertions, 29 deletions

diff --git a/internal/rpn/metric_test.go b/internal/rpn/metric_test.go
index 48c6e91..69923cb 100644
--- a/internal/rpn/metric_test.go
+++ b/internal/rpn/metric_test.go
@@ -586,7 +586,7 @@ func TestNumberDefaultMetricIsCool(t *testing.T) {
 	// NewNumber defaults to Cool
 	n := NewNumber(42, FloatMode)
 	if n.Metric() != cool {
-		t.Errorf("NewNumber().Metric() = %v, want Cool", n.Metric())
+		t.Errorf("NewNumberWithMetric().Metric() = %v, want Cool", n.Metric())
 	}
 }
 
@@ -614,9 +614,9 @@ func TestNewNumberWithMetric(t *testing.T) {
 	reg := GetMetricRegistry()
 	hr, _ := reg.Find("hr")
 
-	n := NewNumber(2, FloatMode, hr)
+	n := NewNumberWithMetric(2, FloatMode, hr)
 	if n.Metric() != hr {
-		t.Errorf("NewNumber(metric).Metric() = %v, want hr", n.Metric())
+		t.Errorf("NewNumberWithMetric(metric).Metric() = %v, want hr", n.Metric())
 	}
 }
 
diff --git a/internal/rpn/number.go b/internal/rpn/number.go
index 39aa93b..3f6f004 100644
--- a/internal/rpn/number.go
+++ b/internal/rpn/number.go
@@ -55,18 +55,27 @@ var _ NumericValue = (*Rat)(nil)
 var _ StackValue = (*StringNum)(nil)
 var _ StackValue = (*Symbol)(nil)
 
-// NewNumber creates a Number from a float64 value with the given metric.
-// The actual type depends on the current calculation mode.
-// If metric is nil or omitted, defaults to Cool.
-func NewNumber(value float64, mode CalculationMode, metric ...*Metric) Number {
-	m := GetCoolMetric()
-	if len(metric) > 0 && metric[0] != nil {
-		m = metric[0]
+// NewNumber creates a Number from a float64 value with the given mode.
+// The actual type depends on the current calculation mode (Float or Rat).
+// The metric defaults to Cool (unitless).
+func NewNumber(value float64, mode CalculationMode) Number {
+	if mode == RationalMode {
+		return NewRat(value)
+	}
+	return NewFloat(value)
+}
+
+// NewNumberWithMetric creates a Number from a float64 value with an explicit metric.
+// The actual type depends on the current calculation mode (Float or Rat).
+// If metric is nil, defaults to Cool.
+func NewNumberWithMetric(value float64, mode CalculationMode, metric *Metric) Number {
+	if metric == nil {
+		metric = GetCoolMetric()
 	}
 	if mode == RationalMode {
-		return NewRatWithMetric(value, m)
+		return NewRatWithMetric(value, metric)
 	}
-	return NewFloatWithMetric(value, m)
+	return NewFloatWithMetric(value, metric)
 }
 
 // GetCoolMetric returns the universal default metric.
diff --git a/internal/rpn/number_test.go b/internal/rpn/number_test.go
index 56ff70b..724e817 100644
--- a/internal/rpn/number_test.go
+++ b/internal/rpn/number_test.go
@@ -432,7 +432,7 @@ func TestNewNumberWithExplicitMetric(t *testing.T) {
 	reg := GetMetricRegistry()
 	mbps, _ := reg.Find("Mbps")
 
-	n := NewNumber(100, FloatMode, mbps)
+	n := NewNumberWithMetric(100, FloatMode, mbps)
 	val, _ := n.Float64()
 	if val != 100 {
 		t.Errorf("Float64() = %v, want 100", val)
diff --git a/internal/rpn/operations_arithmetic.go b/internal/rpn/operations_arithmetic.go
index 3215917..70c048a 100644
--- a/internal/rpn/operations_arithmetic.go
+++ b/internal/rpn/operations_arithmetic.go
@@ -61,7 +61,7 @@ func (o *Operations) binaryMetricOp(
 		return buildError(op, err)
 	}
 
-	stack.Push(NewNumber(resultVal, o.GetMode(), resultMetric))
+	stack.Push(NewNumberWithMetric(resultVal, o.GetMode(), resultMetric))
 	return nil
 }
 
@@ -152,7 +152,7 @@ func (o *Operations) Divide(stack *Stack) error {
 		return buildError("/", err)
 	}
 
-	stack.Push(NewNumber(resultVal, o.GetMode(), resultMetric))
+	stack.Push(NewNumberWithMetric(resultVal, o.GetMode(), resultMetric))
 	return nil
 }
 
@@ -182,7 +182,7 @@ func (o *Operations) Power(stack *Stack) error {
 		return buildError("power", err)
 	}
 
-	stack.Push(NewNumber(math.Pow(aF, bF), o.GetMode(), GetCoolMetric()))
+	stack.Push(NewNumberWithMetric(math.Pow(aF, bF), o.GetMode(), GetCoolMetric()))
 	return nil
 }
 
@@ -231,7 +231,7 @@ func (o *Operations) Modulo(stack *Stack) error {
 		return buildError("%", err)
 	}
 
-	stack.Push(NewNumber(resultVal, o.GetMode(), resultMetric))
+	stack.Push(NewNumberWithMetric(resultVal, o.GetMode(), resultMetric))
 	return nil
 }
 
@@ -265,11 +265,11 @@ func (o *Operations) FastPower(stack *Stack) error {
 
 	// Result is unitless (Cool metric)
 	if exp == 0 {
-		stack.Push(NewNumber(1, o.GetMode(), GetCoolMetric()))
+		stack.Push(NewNumberWithMetric(1, o.GetMode(), GetCoolMetric()))
 		return nil
 	}
 	resultVal := binaryExponentiationFloat(aF, exp)
-	stack.Push(NewNumber(resultVal, o.GetMode(), GetCoolMetric()))
+	stack.Push(NewNumberWithMetric(resultVal, o.GetMode(), GetCoolMetric()))
 	return nil
 }
 
diff --git a/internal/rpn/operations_hyper.go b/internal/rpn/operations_hyper.go
index 6d74a2d..426b471 100644
--- a/internal/rpn/operations_hyper.go
+++ b/internal/rpn/operations_hyper.go
@@ -54,7 +54,7 @@ func (o *Operations) HyperAdd(stack *Stack) error {
 		return buildError("[+]", err)
 	}
 
-	stack.Push(NewNumber(resultVal, o.GetMode(), resultMetric))
+	stack.Push(NewNumberWithMetric(resultVal, o.GetMode(), resultMetric))
 	return nil
 }
 
@@ -83,7 +83,7 @@ func (o *Operations) HyperMultiply(stack *Stack) error {
 	if err != nil {
 		return buildError("[*]", err)
 	}
-	stack.Push(NewNumber(product, o.GetMode(), cool))
+	stack.Push(NewNumberWithMetric(product, o.GetMode(), cool))
 	return nil
 }
 
@@ -134,7 +134,7 @@ func (o *Operations) HyperSubtract(stack *Stack) error {
 		return buildError("[-]", err)
 	}
 
-	stack.Push(NewNumber(resultVal, o.GetMode(), resultMetric))
+	stack.Push(NewNumberWithMetric(resultVal, o.GetMode(), resultMetric))
 	return nil
 }
 
@@ -166,7 +166,7 @@ func (o *Operations) HyperDivide(stack *Stack) error {
 	if err != nil {
 		return buildError("[/]", err)
 	}
-	stack.Push(NewNumber(result, o.GetMode(), cool))
+	stack.Push(NewNumberWithMetric(result, o.GetMode(), cool))
 	return nil
 }
 
@@ -195,7 +195,7 @@ func (o *Operations) HyperPower(stack *Stack) error {
 	if err != nil {
 		return buildError("[^]", err)
 	}
-	stack.Push(NewNumber(result, o.GetMode(), cool))
+	stack.Push(NewNumberWithMetric(result, o.GetMode(), cool))
 	return nil
 }
 
@@ -249,7 +249,7 @@ func (o *Operations) HyperModulo(stack *Stack) error {
 		return buildError("[%]", err)
 	}
 
-	stack.Push(NewNumber(resultVal, o.GetMode(), resultMetric))
+	stack.Push(NewNumberWithMetric(resultVal, o.GetMode(), resultMetric))
 	return nil
 }
 
@@ -277,7 +277,7 @@ func (o *Operations) hyperLog(stack *Stack, opName string, logFn func(float64) f
 	if err != nil {
 		return buildError(opName, err)
 	}
-	stack.Push(NewNumber(result, o.GetMode(), cool))
+	stack.Push(NewNumberWithMetric(result, o.GetMode(), cool))
 	return nil
 }
 
diff --git a/internal/rpn/operations_metric.go b/internal/rpn/operations_metric.go
index 949c0c3..c09aad4 100644
--- a/internal/rpn/operations_metric.go
+++ b/internal/rpn/operations_metric.go
@@ -218,6 +218,6 @@ func (o *Operations) Convert(stack *Stack) error {
 		return buildError("convert", err)
 	}
 	// 6. Push result with target metric
-	stack.Push(NewNumber(resultVal, o.GetMode(), targetMetric))
+	stack.Push(NewNumberWithMetric(resultVal, o.GetMode(), targetMetric))
 	return nil
 }
diff --git a/internal/rpn/rpn_parse.go b/internal/rpn/rpn_parse.go
index ef7be53..eb4f0b9 100644
--- a/internal/rpn/rpn_parse.go
+++ b/internal/rpn/rpn_parse.go
@@ -331,7 +331,7 @@ func (r *RPN) evaluate(input string, tokens []string) (string, error) {
 			if stack.Len() >= r.maxStack {
 				return "", fmt.Errorf("stack overflow")
 			}
-			stack.Push(NewNumber(num, r.mode, metric))
+			stack.Push(NewNumberWithMetric(num, r.mode, metric))
 			continue
 		}
 
@@ -343,7 +343,7 @@ func (r *RPN) evaluate(input string, tokens []string) (string, error) {
 				if stack.Len() >= r.maxStack {
 					return "", fmt.Errorf("stack overflow")
 				}
-				stack.Push(NewNumber(1, r.mode, metric))
+				stack.Push(NewNumberWithMetric(1, r.mode, metric))
 				continue
 			}
 			return "", fmt.Errorf("unknown metric %q in %q", metricName, token)