feat: wire DeleteVariable to OperatorRegistry as 'd' operator

The 'd' token was previously a no-op error. Now it pops a value from
the stack and deletes the variable with that name.

Accepts Symbol (:x) or StringNum as the variable name. Added
DeleteVariable to VariableOperator interface so the operator can
call it through the Operator interface.

Example: x 5 = :x d vars  → No variables defined

3 files changed, 18 insertions, 3 deletions

diff --git a/internal/rpn/operations_interfaces.go b/internal/rpn/operations_interfaces.go
index b6c2baa..0f461f0 100644
--- a/internal/rpn/operations_interfaces.go
+++ b/internal/rpn/operations_interfaces.go
@@ -54,6 +54,7 @@ type VariableOperator interface {
 	ClearVariables()
 	AssignLeft(stack *Stack) error
 	AssignRight(stack *Stack) error
+	DeleteVariable(name string) error
 }
 
 // ConstantOperator defines the interface for constant operations.
diff --git a/internal/rpn/operator_registry.go b/internal/rpn/operator_registry.go
index f2ea64f..9f8e9c8 100644
--- a/internal/rpn/operator_registry.go
+++ b/internal/rpn/operator_registry.go
@@ -54,7 +54,21 @@ func NewOperatorRegistry(op Operator) *OperatorRegistry {
 	registry.registerStandardOperator("swap", func(stack *Stack) error { return op.Swap(stack) })
 	registry.registerStandardOperator("pop", func(stack *Stack) error { return op.Pop(stack) })
 	registry.registerStandardOperator("d", func(stack *Stack) error {
-		return fmt.Errorf("'d' command not supported as standalone token")
+		val, err := popStack(stack, "d")
+		if err != nil {
+			return err
+		}
+		// Extract variable name from the value
+		var name string
+		switch v := val.(type) {
+		case *Symbol:
+			name = v.Name()
+		case *StringNum:
+			name = v.String()
+		default:
+			return fmt.Errorf("delete expects a variable name, got %T", val)
+		}
+		return op.DeleteVariable(name)
 	})
 
 	// Commands that return immediately
diff --git a/internal/rpn/rpn_test.go b/internal/rpn/rpn_test.go
index 494e377..6f7deee 100644
--- a/internal/rpn/rpn_test.go
+++ b/internal/rpn/rpn_test.go
@@ -522,9 +522,9 @@ func TestParseAndEvaluateEvaluateErrors(t *testing.T) {
 			expectedError: "invalid assignment syntax",
 		},
 		{
-			name:          "'d' command not supported",
+			name:          "'d' command without operand",
 			input:         "d",
-			expectedError: "'d' command not supported as standalone token",
+			expectedError: "insufficient operands for d",
 		},
 		{
 			name:          "empty result after evaluation",