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

backend/simple.lisp

@@ -0,0 +1,62 @@
+(defclass simple-backend (backend)
+  ((output-stream :initform *standard-output*
+                  :accessor backend-output-stream)))
+
+(defmethod initialize-backend ((b simple-backend))
+  b)
+
+(defmethod shutdown-backend ((b simple-backend))
+  (values))
+
+(defmethod backend-size ((b simple-backend))
+  ;; Try ioctl, fall back to 80x24
+  (values 80 24))
+
+(defmethod backend-write ((b simple-backend) string)
+  (let ((stream (backend-output-stream b)))
+    (write-string string stream)
+    (finish-output stream)
+    (length string)))
+
+(defmethod draw-text ((b simple-backend) x y string fg bg
+                      &key bold italic underline reverse dim blink)
+  (declare (ignore x y fg bg bold italic underline reverse dim blink))
+  (backend-write b string))
+
+(defun %simple-border-char (edge-style pos)
+  "Return ASCII border character for EDGE-STYLE at POS.
+POS is :top-left, :top-right, :bottom-left, :bottom-right,
+:horizontal, or :vertical."
+  (case pos
+    ((:top-left :top-right :bottom-left :bottom-right) #\+)
+    (:horizontal #\-)
+    (:vertical #\|)))
+
+(defmethod draw-border ((b simple-backend) x y width height
+                        &key style fg bg title title-align)
+  (declare (ignore style fg bg title title-align))
+  (let ((h (%simple-border-char nil :horizontal))
+        (v (%simple-border-char nil :vertical)))
+    ;; Top edge
+    (backend-write b (format nil "~%~v@{~a~:*~}" width h))
+    ;; Sides
+    (loop for i from 1 below (1- height)
+          do (backend-write b (format nil "~%|~v@{~a~:*~}|" (- width 2) #\space)))
+    ;; Bottom edge
+    (backend-write b (format nil "~%~v@{~a~:*~}" width h))))
+
+(defmethod draw-rect ((b simple-backend) x y width height
+                      &key bg)
+  (declare (ignore x y width height bg))
+  ;; On simple backend, background fill is a no-op
+  (values))
+
+(defmethod draw-link ((b simple-backend) x y string url
+                      &key fg bg)
+  (declare (ignore url fg bg))
+  (draw-text b x y string nil nil))
+
+(defmethod draw-ellipsis ((b simple-backend) x y width
+                          &key fg bg)
+  (declare (ignore x y width fg bg))
+  (backend-write b "..."))