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/classes.lisp

@@ -1,3 +1,5 @@
+(in-package :cl-tui.backend)
+
 (defclass backend () ())
 
 (defgeneric initialize-backend (backend)