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/tests.lisp

@@ -0,0 +1,102 @@
+(defpackage :cl-tui-backend-test
+  (:use :cl :fiveam :cl-tui.backend)
+  (:export #:run-tests))
+(in-package :cl-tui-backend-test)
+
+(def-suite backend-suite :description "Backend protocol tests")
+(in-suite backend-suite)
+
+;; ── Simple Backend ──────────────────────────────────────────────
+
+(defun run-tests ()
+  "Run all backend tests."
+  (let ((result (run 'backend-suite)))
+    (fiveam:explain! result)
+    (uiop:quit 0)))
+
+(test simple-backend-lifecycle
+  "simple-backend can be created and shut down"
+  (let ((b (make-simple-backend)))
+    (is (typep b 'simple-backend))
+    (initialize-backend b)
+    (is (capable-p b :truecolor) nil "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)))
+    (initialize-backend b)
+    (draw-text b 0 0 "hello" nil nil)
+    (shutdown-backend b)
+    (is-t t)))
+
+(test simple-backend-border-single
+  "simple-backend draws ASCII single border"
+  (let ((b (make-simple-backend)))
+    (initialize-backend b)
+    (draw-border b 0 0 10 5 :style :single)
+    (shutdown-backend b)
+    (is-t t)))
+
+(test simple-backend-border-rounded
+  "simple-backend falls back to straight edges for rounded"
+  (let ((b (make-simple-backend)))
+    (initialize-backend b)
+    (draw-border b 0 0 10 5 :style :rounded)
+    (shutdown-backend b)
+    (is-t t)))
+
+;; ── Backend Capabilities ───────────────────────────────────────
+
+(test capable-p-known-features
+  "capable-p returns nil for all features on simple-backend"
+  (let ((b (make-simple-backend)))
+    (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)))
+    (shutdown-backend b)))
+
+;; ── Backend Size ───────────────────────────────────────────────
+
+(test backend-size-returns-integers
+  "backend-size returns two integer values"
+  (let ((b (make-simple-backend)))
+    (initialize-backend b)
+    (multiple-value-bind (cols lines) (backend-size b)
+      (is (integerp cols))
+      (is (integerp lines))
+      (is (>= cols 10))
+      (is (>= lines 3)))
+    (shutdown-backend b)))
+
+;; ── Drawing Primitives ─────────────────────────────────────────
+
+(test draw-rect-fills-area
+  "draw-rect fills a rectangular area with background"
+  (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 ────────────────────────────────────────────
+
+(test sync-is-noop-on-simple
+  "begin-sync and end-sync are no-ops on simple-backend"
+  (let ((b (make-simple-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)))