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/tests.lisp

@@ -6,6 +6,14 @@
 (def-suite backend-suite :description "Backend protocol tests")
 (in-suite backend-suite)
 
+;; ── Helpers ─────────────────────────────────────────────────────
+
+(defun make-capturing-backend ()
+  "Create a simple-backend that writes to a string stream."
+  (let* ((s (make-string-output-stream))
+         (b (make-simple-backend :output-stream s)))
+    (values b s)))
+
 ;; ── Simple Backend ──────────────────────────────────────────────
 
 (defun run-tests ()
@@ -19,32 +27,55 @@
   (let ((b (make-simple-backend)))
     (is (typep b 'simple-backend))
     (initialize-backend b)
-    (is (capable-p b :truecolor) nil "simple backend has no truecolor")
+    (is-false (capable-p b :truecolor) "simple backend has no truecolor")
     (shutdown-backend b)))
 
 (test simple-backend-draw-text
   "simple-backend renders text at position, ignoring style"
-  (let ((b (make-simple-backend)))
+  (multiple-value-bind (b s) (make-capturing-backend)
     (initialize-backend b)
-    (draw-text b 0 0 "hello" nil nil)
+    (draw-text b 0 0 "hello" :red nil :bold t :italic t)
     (shutdown-backend b)
-    (is-t t)))
+    (is (string= (get-output-stream-string s) "hello")
+        "draw-text should output the string ignoring style")))
 
-(test simple-backend-border-single
-  "simple-backend draws ASCII single border"
-  (let ((b (make-simple-backend)))
+(test simple-backend-draw-border
+  "simple-backend draws ASCII border with +-| characters"
+  (multiple-value-bind (b s) (make-capturing-backend)
     (initialize-backend b)
-    (draw-border b 0 0 10 5 :style :single)
+    (draw-border b 0 0 5 3 :style :single)
     (shutdown-backend b)
-    (is-t t)))
+    (let ((out (get-output-stream-string s)))
+      (is (search "-----" out) "top edge should have 5 dashes")
+      (is (search "|   |" out) "middle row should have pipe sides"))))
 
-(test simple-backend-border-rounded
-  "simple-backend falls back to straight edges for rounded"
-  (let ((b (make-simple-backend)))
+(test simple-backend-draw-rounded
+  "simple-backend falls back to straight edges for rounded style"
+  (multiple-value-bind (b s) (make-capturing-backend)
     (initialize-backend b)
-    (draw-border b 0 0 10 5 :style :rounded)
+    (draw-border b 0 0 5 3 :style :rounded)
     (shutdown-backend b)
-    (is-t t)))
+    (let ((out (get-output-stream-string s)))
+      ;; Rounded falls back to ASCII — identical output to single
+      (is (search "-----" out) "rounded style produces same dashes as single"))))
+
+(test simple-backend-draw-link
+  "simple-backend renders link as plain text"
+  (multiple-value-bind (b s) (make-capturing-backend)
+    (initialize-backend b)
+    (draw-link b 0 0 "click me" "http://example.com")
+    (shutdown-backend b)
+    (is (string= (get-output-stream-string s) "click me")
+        "simple-backend ignores URL, outputs text only")))
+
+(test simple-backend-draw-ellipsis
+  "simple-backend renders ... for ellipsis"
+  (multiple-value-bind (b s) (make-capturing-backend)
+    (initialize-backend b)
+    (draw-ellipsis b 0 0 5)
+    (shutdown-backend b)
+    (is (string= (get-output-stream-string s) "...")
+        "ellipsis should output 3 dots")))
 
 ;; ── Backend Capabilities ───────────────────────────────────────
 
@@ -54,8 +85,8 @@
     (initialize-backend b)
     (dolist (f '(:truecolor :osc8 :sync :mouse :bracketed-paste
                  :kitty-keyboard :sixel :cursor-style))
-      (is (capable-p b f) nil
-          (format nil "~s should not be supported on simple-backend" f)))
+      (is-false (capable-p b f)
+                (format nil "~s should not be supported on simple-backend" f)))
     (shutdown-backend b)))
 
 ;; ── Backend Size ───────────────────────────────────────────────
@@ -71,32 +102,37 @@
       (is (>= lines 3)))
     (shutdown-backend b)))
 
-;; ── Drawing Primitives ─────────────────────────────────────────
+;; ── Backend Protocol: Defaults and No-ops ──────────────────────
 
-(test draw-rect-fills-area
-  "draw-rect fills a rectangular area with background"
+(test default-methods-are-no-ops
+  "Default backend methods don't error"
   (let ((b (make-simple-backend)))
     (initialize-backend b)
-    (draw-rect b 0 0 5 3 :bg nil)
-    (shutdown-backend b)
-    (is-t t)))
-
-(test draw-text-multi-line
-  "draw-text handles strings with newlines"
-  (let ((b (make-simple-backend)))
-    (initialize-backend b)
-    (draw-text b 0 0 "line1~%line2" nil nil)
-    (shutdown-backend b)
-    (is-t t)))
-
-;; ── Synchronization ────────────────────────────────────────────
+    (is (null (multiple-value-list (cursor-hide b))))
+    (is (null (multiple-value-list (cursor-show b))))
+    (is (null (multiple-value-list (cursor-style b :block))))
+    (is (null (multiple-value-list (begin-sync b))))
+    (is (null (multiple-value-list (end-sync b))))
+    (shutdown-backend b)))
 
 (test sync-is-noop-on-simple
-  "begin-sync and end-sync are no-ops on simple-backend"
-  (let ((b (make-simple-backend)))
+  "begin-sync and end-sync produce no output on simple-backend"
+  (multiple-value-bind (b s) (make-capturing-backend)
     (initialize-backend b)
     (begin-sync b)
     (draw-text b 0 0 "in sync" nil nil)
     (end-sync b)
     (shutdown-backend b)
-    (is-t t)))
+    (is (string= (get-output-stream-string s) "in sync")
+        "no sync escape sequences should appear")))
+
+;; ── Draw-rect ──────────────────────────────────────────────────
+
+(test draw-rect-fills-area-correctly
+  "draw-rect with background writes nothing to output (simple-backend no-op)"
+  (multiple-value-bind (b s) (make-capturing-backend)
+    (initialize-backend b)
+    (draw-rect b 0 0 5 3 :bg :red)
+    (shutdown-backend b)
+    (is (string= (get-output-stream-string s) "")
+        "draw-rect is a no-op on simple-backend")))