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%.

src/components/markdown.lisp

@@ -2,8 +2,6 @@
 
 (in-package :cl-tty.markdown)
 
-;; ─── Node constructors ────────────────────────────────────────────────────────
-
 (defun make-md-node (type &key children properties content url)
   (let ((node (list :type type)))
     (when children (setf (getf node :children) children))
@@ -28,8 +26,6 @@
                   (mapcar #'md-node-text (getf node :children))))
           (t ""))))
 
-;; ─── Block-level parser ───────────────────────────────────────────────────────
-
 (defun split-string-into-lines (string)
   (unless string (return-from split-string-into-lines (coerce nil 'vector)))
   (let ((result nil) (start 0))
@@ -250,8 +246,6 @@
                  (t (incf i)))))
     (nreverse nodes)))
 
-;; ─── Inline parser ────────────────────────────────────────────────────────────
-
 (defun parse-inline (text)
   (unless (and text (> (length text) 0)) (return-from parse-inline nil))
   (let ((nodes nil) (i 0) (len (length text)))
@@ -348,8 +342,6 @@
                             :url (subseq text (+ close-bracket 2) close-paren))
               (1+ close-paren)))))
 
-;; ─── Syntax highlighting ──────────────────────────────────────────────────────
-
 (defun get-highlighter (lang)
   (cdr (assoc lang
               '(("lisp"    . (:comment (";" "#|" ";;") :string ("\"")
@@ -525,8 +517,6 @@
 (defun apply-highlight-style (char-vector)
   (coerce char-vector 'string))
 
-;; ─── Diff rendering ───────────────────────────────────────────────────────────
-
 (defun string-prefix-p (prefix string)
   (and (>= (length string) (length prefix))
        (string= prefix (subseq string 0 (length prefix)))))
@@ -539,8 +529,6 @@
         ((string-prefix-p "-" line) :removed)
         (t :context)))
 
-;; ─── Rendering ────────────────────────────────────────────────────────────────
-
 (defun apply-style (style text)
   (let ((code (cond
                ((eql style :bold) "1") ((eql style :italic) "3")