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/modern.lisp

@@ -111,7 +111,7 @@
                  (case pos (:horizontal "─") (:vertical "│"))
                  "+"))))
 
-(defclass modern-backend ()
+(defclass modern-backend (backend)
   ((output-stream :initform *standard-output*
                   :accessor backend-output-stream)
    (in-sync-p :initform nil :accessor in-sync-p)))
@@ -199,14 +199,14 @@
     (backend-write b bot)))
 
 (defmethod draw-rect ((b modern-backend) x y width height &key bg)
-  (let ((bg-esc (sgr-bg bg))
-        (reset (sgr-attr :reset))
-        (line (concatenate 'string
-                bg-esc
-                (make-string width :initial-element #\Space)
-                reset (string #\Newline))))
-    (loop repeat height do
-      (backend-write b (cursor-move-escape x y))
+  (let* ((bg-esc (sgr-bg bg))
+         (reset (sgr-attr :reset))
+         (line (concatenate 'string
+                 bg-esc
+                 (make-string width :initial-element #\Space)
+                 reset (string #\Newline))))
+    (loop :for row :from 0 :below height :do
+      (backend-write b (cursor-move-escape x (+ y row)))
       (backend-write b line))))
 
 (defmethod draw-link ((b modern-backend) x y string url