fix(rpn): clean up metric subcommands per review feedback

- Remove dead 'i += 2' in metric binary/decimal set handlers
  (the increment is followed by immediate return, never used)
- Clarify MetricShow always uses SI factor in display
  (prefixMode not yet wired into computations)
- Add test for metric Custom category

3 files changed, 13 insertions, 3 deletions

diff --git a/internal/rpn/operations_metric_cmd.go b/internal/rpn/operations_metric_cmd.go
index 9a09a67..57303a6 100644
--- a/internal/rpn/operations_metric_cmd.go
+++ b/internal/rpn/operations_metric_cmd.go
@@ -22,7 +22,7 @@ func (o *Operations) MetricShow(stack *Stack) (string, error) {
 	if m == nil || m.Category == Universal {
 		return "Cool (Universal)", nil
 	}
-	factor := m.Factor(SI) // use SI for display
+	factor := m.Factor(SI) // display uses SI; prefixMode is not yet wired into computations
 	return fmt.Sprintf("%s, %s, base: %s, factor: %.0g", m.Name, m.Category, m.BaseUnit, factor), nil
 }
 
diff --git a/internal/rpn/operations_metric_cmd_test.go b/internal/rpn/operations_metric_cmd_test.go
index 9b77439..a2bfcca 100644
--- a/internal/rpn/operations_metric_cmd_test.go
+++ b/internal/rpn/operations_metric_cmd_test.go
@@ -103,6 +103,18 @@ func TestMetricCategoryUnknown(t *testing.T) {
 	}
 }
 
+func TestMetricCategoryCustom(t *testing.T) {
+	vars := NewVariables()
+	rpn := NewRPN(vars)
+	// Custom category exists but has no built-in metrics, so result should be empty
+	result, err := rpn.ParseAndEvaluate("metric Custom")
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	// Empty string is fine — no custom metrics registered by default
+	_ = result
+}
+
 func TestMetricSetModeDecimal(t *testing.T) {
 	vars := NewVariables()
 	rpn := NewRPN(vars)
diff --git a/internal/rpn/rpn_parse.go b/internal/rpn/rpn_parse.go
index 5d5ef7d..bc25d52 100644
--- a/internal/rpn/rpn_parse.go
+++ b/internal/rpn/rpn_parse.go
@@ -368,14 +368,12 @@ func (r *RPN) evaluate(input string, tokens []string) (string, error) {
 			case "binary":
 				if i+2 < len(tokens) && tokens[i+2] == "set" {
 					r.prefixMode = IEC
-					i += 2 // skip "binary" and "set"
 					return "prefix mode: IEC", nil
 				}
 				return "", fmt.Errorf("rpn: metric binary: use 'metric binary set'")
 			case "decimal":
 				if i+2 < len(tokens) && tokens[i+2] == "set" {
 					r.prefixMode = SI
-					i += 2 // skip "decimal" and "set"
 					return "prefix mode: SI", nil
 				}
 				return "", fmt.Errorf("rpn: metric decimal: use 'metric decimal set'")