refactor: fix parseCategory OCP violation with range-based iteration

Replace the switch/case in parseCategory() with range-based iteration
over all Category constants (Universal through _sentinel). This achieves
true OCP compliance: adding a new Category constant before _sentinel
automatically makes it available in parseCategory without modifying the
function.

Added _sentinel constant as the upper bound for range-based iteration,
with a comment explaining how to add new categories correctly.

2 files changed, 12 insertions, 19 deletions

diff --git a/internal/rpn/metric_type.go b/internal/rpn/metric_type.go
index b9a9514..1a9c9f5 100644
--- a/internal/rpn/metric_type.go
+++ b/internal/rpn/metric_type.go
@@ -25,6 +25,10 @@ const (
 	Distance
 	// Custom is for user-defined units.
 	Custom
+	// _sentinel marks the upper bound for range-based Category iteration.
+	// Adding a new Category: insert before _sentinel, and it will be
+	// automatically picked up by parseCategory() and other range-based lookups.
+	_sentinel
 )
 
 // String returns the human-readable name of the category.
diff --git a/internal/rpn/operations_metric_cmd.go b/internal/rpn/operations_metric_cmd.go
index b8daa1a..868c92a 100644
--- a/internal/rpn/operations_metric_cmd.go
+++ b/internal/rpn/operations_metric_cmd.go
@@ -81,27 +81,16 @@ func (o *Operations) MetricCompatible(stack *Stack) (string, error) {
 }
 
 // parseCategory converts a category name string to a Category constant.
+// Iterates over all valid Category values using range, so adding a new
+// Category constant (between Universal and _sentinel) automatically makes
+// it available here without modifying this function (OCP compliance).
 func parseCategory(name string) (Category, bool) {
-	switch name {
-	case "Universal":
-		return Universal, true
-	case "DataRate":
-		return DataRate, true
-	case "DataSize":
-		return DataSize, true
-	case "Time":
-		return Time, true
-	case "Weight":
-		return Weight, true
-	case "Speed":
-		return Speed, true
-	case "Distance":
-		return Distance, true
-	case "Custom":
-		return Custom, true
-	default:
-		return 0, false
+	for cat := Category(0); cat <= _sentinel; cat++ {
+		if cat.String() == name {
+			return cat, true
+		}
 	}
+	return 0, false
 }
 
 // CustomList returns all custom metric names.