v0.0.1: backend protocol — abstraction layer + simple backend

Implement the backend protocol with two backends (modern planned,
simple done). Includes package definitions, CLOS generic protocol,
simple-backend with ASCII borders, and 9 FiveAM tests.

RED: 9/9 tests failing (no implementation)
GREEN: 9/9 tests passing

- backend/package.lisp — defpackage, exports
- backend/classes.lisp — backend base class, 18 generics
- backend/simple.lisp — simple-backend implementation
- backend/tests.lisp — 9 FiveAM test cases
- org/backend-protocol.org — literate source

cl-tui.asd

@@ -0,0 +1,24 @@
+;;; cl-tui.asd — Common Lisp Terminal UI Framework
+(asdf:defsystem :cl-tui
+  :description "Reusable Common Lisp Terminal UI Framework"
+  :author "Amr Gharbeia"
+  :version "0.0.1"
+  :license "TBD"
+  :depends-on (:fiveam)
+  :components
+  ((:module "backend"
+    :components
+    ((:file "package")
+     (:file "classes" :depends-on ("package"))
+     (:file "simple" :depends-on ("package" "classes")))))
+  :in-order-to ((test-op (test-op :cl-tui-tests))))
+
+(asdf:defsystem :cl-tui-tests
+  :description "Test suite for cl-tui"
+  :depends-on (:cl-tui :fiveam)
+  :components
+  ((:module "backend"
+    :components
+    ((:file "tests"))))
+  :perform (test-op (o c)
+             (uiop:symbol-call :cl-tui-backend-test '#:run!)))