v1.0.0 review fixes: dialog, textarea, scrollbox, demo, ASDF, layout

Fixes from subagent code review (15 findings):

CRITICAL runtime bugs:
- dialog.lisp: backend-write calls -> draw-rect/draw-text (wrong arg count)
- dialog.lisp: removed undefined render-component call
- dialog.lisp: toast render backend-write -> draw-text

MAJOR data loss / silent failures:
- textarea.lisp: undo overflow now drops oldest entry instead of wiping stack
- scrollbox.lisp: :background-element -> :bright-black (theme keyword never resolved)

ASDF completeness:
- modern-tests.lisp wired as component and test-op suite
- layout tests added to test-op suite list
- markdown suite lookup now uses keyword (was looking up wrong string)
- test runner updated to match

API cleanup:
- container-package: removed duplicate render export
- select-package: removed duplicate render export
- markdown.lisp: #\Escape -> #\Esc for consistency
- textarea.lisp: removed duplicate %split-string defn

Demo robustness:
- Added unwind-protect for guaranteed terminal cleanup
- Uses make-modern-backend constructor
- Uses set-raw-mode/restore-terminal-state

Layout:
- normalize-box handles partial padding specs (was returning all zeros)

cl-tty.asd

@@ -56,7 +56,8 @@
   :components
   ((:module "backend"
     :components
-    ((:file "tests")))
+    ((:file "tests")
+     (:file "modern-tests" :depends-on ("tests"))))
    (:module "layout"
     :components
     ((:file "tests")))
@@ -81,12 +82,17 @@
                                 (:cl-tty-input-test "INPUT-SUITE")
                                 (:cl-tty-scrollbox-test "SCROLLBOX-SUITE")
                                 (:cl-tty-select-test "SELECT-SUITE")
-                                (:cl-tty-markdown-test "MARKDOWN-SUITE")
+                                (:cl-tty-markdown-test)
                                 (:cl-tty-dialog-test "DIALOG-SUITE")
                                 (:cl-tty-mouse-test "MOUSE-SUITE")
-                                (:cl-tty-slot-test "SLOT-SUITE")))
+                                (:cl-tty-slot-test "SLOT-SUITE")
+                                (:cl-tty-layout-test "LAYOUT-SUITE")
+                                (:cl-tty-modern-backend-test "MODERN-BACKEND-SUITE")))
                    (let* ((pkg (find-package (first suite)))
-                          (s (and pkg (find-symbol (second suite) pkg))))
+                          (suite-name (second suite))
+                          (s (cond (suite-name (find-symbol suite-name pkg))
+                                   (pkg (find-symbol (string (first suite)) :keyword))
+                                   (t nil))))
                      (when s
                        (funcall explain (funcall run s))))))
                (uiop:quit 0)))