literate: restructure all 19 org files with per-function blocks and prose

Every function, defclass, defstruct, defgeneric, defmethod, defmacro,
defvar, and defparameter in every org file now has its own #+BEGIN_SRC
block with literate prose above it explaining the design reasoning.

Block counts before → after:
  package.org:           1 → 7
  container-package.org: 1 → 1 (prose expanded)
  dirty.org:             4 → 6
  render.org:           10 → 25
  theme.org:             6 → 19
  box-renderable.org:    9 → 29
  scrollbox.org:         8 → 26
  tabbar.org:            5 → 10
  backend-protocol.org:  8 → 66
  modern-backend.org:   17 → 53
  detection.org:         4 → 6
  layout-engine.org:     9 → 36
  framebuffer.org:       8 → 37
  markdown-renderer.org:13 → 38
  dialog.org:           17 → 23 (merged dual structure)
  mouse.org:             4 → 25
  select.org:           12 → 30
  slot.org:              4 → 12
  text-input.org:       11 → 53

Total: ~153 blocks → ~502 blocks

Bugs fixed during restructuring:
- render.org: stray π character typo (backenπd → backend)
- modern-backend.org: sgr-attr missing closing paren + #+END_SRC
- detection.org: invalid #\Esc character reference
- select.org: extra closing paren in select-visible-options

All 13 test suites pass at 100%.

tests/input-tests.lisp

@@ -190,14 +190,11 @@
     (handle-textarea-input a (make-key-event :key :a :code (char-code #\a)))
     (handle-textarea-input a (make-key-event :key :enter))
     (handle-textarea-input a (make-key-event :key :b :code (char-code #\b)))
-    (is (string= (textarea-value a) "a
-b"))))
+    (is (string= (textarea-value a) (format nil "a~Cb" #\Newline)))))
 
 (test textarea-cursor-up-down
   "Cursor moves between lines maintaining column position."
-  (let ((a (make-textarea :value "abc
-de
-fghi")))
+  (let ((a (make-textarea :value (format nil "abc~Cde~Cfghi" #\Newline #\Newline))))
     (setf (textarea-cursor-row a) 1)
     (setf (textarea-cursor-col a) 1)
     (handle-textarea-input a (make-key-event :key :up))
@@ -209,8 +206,7 @@ fghi")))
 
 (test textarea-cursor-up-down-bounds
   "Cursor cannot move past first or last line."
-  (let ((a (make-textarea :value "a
-b")))
+  (let ((a (make-textarea :value (format nil "a~Cb" #\Newline))))
     (handle-textarea-input a (make-key-event :key :up))
     (is (= (textarea-cursor-row a) 0))
     (setf (textarea-cursor-row a) 1)
@@ -219,8 +215,7 @@ b")))
 
 (test textarea-backspace-joins-lines
   "Backspace at start of a line joins with previous."
-  (let ((a (make-textarea :value "hello
-world")))
+  (let ((a (make-textarea :value (format nil "hello~Cworld" #\Newline))))
     (setf (textarea-cursor-row a) 1)
     (setf (textarea-cursor-col a) 0)
     (handle-textarea-input a (make-key-event :key :backspace))

tests/mouse-tests.lisp

@@ -18,8 +18,6 @@
   (setf cl-tty.mouse::*selection* (make-selection :text "hello"))
   (is (equal "hello" (get-selection))))
 
-;; ── Selection tracking ──────────────────────────────────────
-
 (def-test start-selection-initializes-state ()
   (start-selection 5 10)
   (is-true (selection-active-p))

tests/scrollbox-tabbar-tests.lisp

@@ -11,8 +11,6 @@
     (fiveam:explain! result)
     (uiop:quit 0)))
 
-;; ── ScrollBox Tests ─────────────────────────────────────────────
-
 (test scrollbox-creates
   "A ScrollBox can be created with defaults."
   (let ((sb (make-scroll-box)))
@@ -46,8 +44,6 @@
     (render sb backend)
     (is-true t)))
 
-;; ── TabBar Tests ────────────────────────────────────────────────
-
 (test tabbar-creates
   "A TabBar can be created with defaults."
   (let ((tb (make-tab-bar)))