feat(rpn): show metric suffix in stack display

Task 9d: Operations.Show() now appends metric suffix for non-Cool
metrics (e.g., '100Mbps 42 5.5Mbps'). Cool numbers display as plain
numbers. Added TestShowWithMetrics.

2 files changed, 47 insertions, 4 deletions

diff --git a/internal/rpn/operations_stack.go b/internal/rpn/operations_stack.go
index 2505e37..e8752a6 100644
--- a/internal/rpn/operations_stack.go
+++ b/internal/rpn/operations_stack.go
@@ -52,6 +52,8 @@ func (o *Operations) Pop(stack *Stack) error {
 }
 
 // Show returns the current stack as a formatted string using the Number interface.
+// Numbers with non-Cool metrics display with the metric suffix (e.g., "100Mbps").
+// Cool numbers display as plain numbers. Booleans show as "true"/"false".
 func (o *Operations) Show(stack *Stack) (string, error) {
 	if stack.Len() == 0 {
 		return "Stack is empty", nil
@@ -63,10 +65,13 @@ func (o *Operations) Show(stack *Stack) (string, error) {
 		if i > 0 {
 			result += " "
 		}
-		// Use val.String() to format values correctly:
-		// - Boolean values show as "true"/"false"
-		// - Number values show with appropriate precision
-		result += val.String()
+		// Append metric suffix for non-Cool metrics
+		m := val.Metric()
+		if m != nil && m.Category != Universal {
+			result += val.String() + m.Name
+		} else {
+			result += val.String()
+		}
 	}
 	return result, nil
 }
diff --git a/internal/rpn/operations_stack_test.go b/internal/rpn/operations_stack_test.go
new file mode 100644
index 0000000..f31357b
--- /dev/null
+++ b/internal/rpn/operations_stack_test.go
@@ -0,0 +1,38 @@
+// SPDX-License-Identifier: MIT
+// Copyright (c) 2026 Paul Buetow
+
+package rpn
+
+import (
+	"strings"
+	"testing"
+)
+
+func TestShowWithMetrics(t *testing.T) {
+	reg := GetMetricRegistry()
+	vars := NewVariables()
+	ops := NewOperations(vars)
+	stack := NewStack()
+
+	mbps, _ := reg.Find("Mbps")
+	cool, _ := reg.Find("Cool")
+
+	stack.Push(NewFloatWithMetric(100, mbps))
+	stack.Push(NewFloatWithMetric(42, cool))
+	stack.Push(NewFloatWithMetric(5.5, mbps))
+
+	result, err := ops.Show(stack)
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	// Non-Cool shows metric suffix, Cool doesn't
+	if !strings.Contains(result, "100Mbps") {
+		t.Errorf("expected '100Mbps' in result, got: %s", result)
+	}
+	if !strings.Contains(result, "42") {
+		t.Errorf("expected '42' (plain) in result, got: %s", result)
+	}
+	if !strings.Contains(result, "5.5Mbps") {
+		t.Errorf("expected '5.5Mbps' in result, got: %s", result)
+	}
+}