fix: replace fragile string matching in ResultStack with registry check

ResultStack used strings.Contains(err.Error(), "unknown token") to
decide whether to fall through to variable lookup. This is fragile and
breaks if error messages change. Use the same pattern as handleOperator:
check IsStandardOperator/IsHyperOperator directly.

1 files changed, 2 insertions, 3 deletions

diff --git a/internal/rpn/rpn_ops.go b/internal/rpn/rpn_ops.go
index 9104eea..9208195 100644
--- a/internal/rpn/rpn_ops.go
+++ b/internal/rpn/rpn_ops.go
@@ -34,9 +34,8 @@ func (r *RPN) ResultStack(tokens []string) (string, error) {
 
 		// Handle operator (common logic from executeOperator)
 		if result, handled, err := r.executeOperator(stack, token); err != nil {
-			// If the error is not "unknown token", return it
-			// Otherwise, fall through to check for variable
-			if !strings.Contains(err.Error(), "unknown token") {
+			// If not a recognized operator, fall through to check for variable
+			if r.opRegistry.IsStandardOperator(token) || r.opRegistry.IsHyperOperator(token) {
 				return "", err
 			}
 		} else if handled {