review fixes: 3 blocking bugs + 2 improvements

B1: modern-backend now inherits from backend (was standalone class)
B2: draw-rect y-position bug — loop now tracks row offset
B3: Layout module added to ASDF system definition
I1: 6 smoke tests replaced with behavioral tests (captured output)
I3: 6 edge case tests: empty, single-child, zero-size, deep
    nesting, large padding, negative grow

Also fixed:
- Added missing make-simple-backend constructor to simple.lisp
- Added in-package to classes.lisp and simple.lisp (SBCL's load
  restores *package* after each load, breaking batch-mode loading)

backend/simple.lisp

@@ -1,7 +1,14 @@
+(in-package :cl-tui.backend)
+
 (defclass simple-backend (backend)
   ((output-stream :initform *standard-output*
+                  :initarg :output-stream
                   :accessor backend-output-stream)))
 
+(defun make-simple-backend (&key output-stream)
+  (make-instance 'simple-backend
+    :output-stream (or output-stream *standard-output*)))
+
 (defmethod initialize-backend ((b simple-backend))
   b)