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)

layout/tests.lisp

@@ -118,3 +118,58 @@
     (let ((sc (layout-node-children sidebar)))
       (is (= (layout-node-y (elt sc 0)) 0))
       (is (= (layout-node-y (elt sc 1)) 3)))))
+
+;; ── Edge Cases ────────────────────────────────────────────────
+
+(test empty-container-does-not-crash
+  "compute-layout on a node with no children should not error"
+  (let ((r (make-layout-node)))
+    (compute-layout r 20 20)
+    (is (integerp (layout-node-width r)))
+    (is (integerp (layout-node-height r)))))
+
+(test single-child-in-column
+  "A column with one child places it correctly"
+  (let* ((r (make-layout-node :direction :column :width 10 :height 20))
+         (c (make-layout-node :height 5)))
+    (layout-node-add-child r c)
+    (compute-layout r 10 20)
+    (is (= (layout-node-y c) 0))
+    (is (= (layout-node-height c) 5))))
+
+(test zero-size-container
+  "compute-layout with zero available space should not error"
+  (let* ((r (make-layout-node :direction :column))
+         (c (make-layout-node :height 5)))
+    (layout-node-add-child r c)
+    (compute-layout r 0 0)
+    (is (integerp (layout-node-x c)))
+    (is (integerp (layout-node-y c)))))
+
+(test deep-nesting-three-levels
+  "Three-level deep nesting produces correct leaf positions"
+  (let* ((out (vbox ()                                    ; outer box
+                (vbox (:grow 1)                           ; middle box
+                  (make-layout-node :height 2))))         ; leaf
+         (leaf (elt (layout-node-children
+                      (elt (layout-node-children out) 0)) 0)))
+    (compute-layout out 20 20)
+    (is (= (layout-node-y leaf) 0))))
+
+(test large-padding-leaves-room
+  "Large padding reduces content area but doesn't crash"
+  (let* ((r (make-layout-node :direction :column
+                              :padding '(:top 5 :left 5 :bottom 5 :right 5)))
+         (c (make-layout-node :height 3)))
+    (layout-node-add-child r c)
+    (compute-layout r 20 20)
+    (is (= (layout-node-x c) 5))
+    (is (= (layout-node-y c) 5))))
+
+(test negative-grow-is-clamped
+  "Grow values are adjusted but still compute"
+  (let* ((r (make-layout-node :direction :row :width 10))
+         (c (make-layout-node :width 5 :grow -1)))
+    (layout-node-add-child r c)
+    (compute-layout r 10 10)
+    (is (integerp (layout-node-width c)))))