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/rendering/framebuffer.lisp

@@ -12,8 +12,6 @@
 
 (in-package :cl-tty.rendering)
 
-;;; ─── Cell — immutable per-cell state ─────────────────────────────────────────
-
 (defstruct cell
   "A single terminal cell — character, colors, and attributes."
   (char #\space :type character)
@@ -24,8 +22,6 @@
   (underline nil :type boolean)
   (link-url nil))
 
-;;; ─── Framebuffer — 2D array of cells ────────────────────────────────────────
-
 (defun make-framebuffer (width height)
   "Create a 2D array of CELL with dimensions HEIGHT x WIDTH."
   (make-array (list height width)
@@ -40,8 +36,6 @@
   "Return the height (rows) of framebuffer FB."
   (if (arrayp fb) (array-dimension fb 0) 0))
 
-;;; ─── Framebuffer Backend — implements backend protocol ─────────────────────
-
 (defclass framebuffer-backend (backend)
   ((framebuffer :initform nil :accessor fb-framebuffer)
    (scissor-x :initform 0 :accessor fb-scissor-x)
@@ -55,8 +49,6 @@
     (setf (fb-framebuffer fb) (make-framebuffer width height))
     fb))
 
-;;; ─── Drawing methods ─────────────────────────────────────────────────────────
-
 (defun %in-scissor-p (fb cx cy)
   "Check if (CX, CY) falls within the current scissor rectangle."
   (let ((sx (fb-scissor-x fb)) (sy (fb-scissor-y fb))
@@ -129,8 +121,6 @@
   (dotimes (i (min 3 width))
     (%set-cell fb (+ x i) y #\. :fg fg :bg bg)))
 
-;;; ─── Diff ────────────────────────────────────────────────────────────────────
-
 (defun cells-equal-p (a b)
   "Return T if two cells have identical content and style."
   (and (eql (cell-char a) (cell-char b))
@@ -153,8 +143,6 @@
             (push (list x y b) changes)))))
     (nreverse changes)))
 
-;;; ─── Flush ───────────────────────────────────────────────────────────────────
-
 (defun flush-framebuffer (prev-fb curr-fb backend)
   "Diff PREV-FB and CURR-FB and flush changes to BACKEND.
 Returns the number of changed cells."
@@ -176,8 +164,6 @@ Returns the number of changed cells."
       (end-sync backend))
     count))
 
-;;; --- Frame inspection ---------------------------------------------------
-
 (defun fb-cell-link-url (fb x y)
   "Return the link URL at (X Y) in framebuffer FB, or nil."
   (when (and (arrayp fb) (>= y 0) (< y (array-dimension fb 0))
@@ -198,8 +184,6 @@ Returns the number of changed cells."
                           (princ (cell-char c) s)))
                (when (< y y-max) (princ #\Newline s))))))
 
-;;; ─── Scissor clipping ────────────────────────────────────────────────────────
-
 (defmacro with-scissor ((fb x y w h) &body body)
   "Clip all drawing on FB to rectangle (X Y W H)."
   (let ((old-x (gensym)) (old-y (gensym))