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/box-tests.lisp

@@ -16,8 +16,6 @@
          (b (make-modern-backend :output-stream s)))
     (values b s)))
 
-;; ── Box Tests ─────────────────────────────────────────────────
-
 (test box-creates-with-defaults
   "A box created with no arguments has reasonable defaults"
   (let ((b (make-box)))
@@ -92,8 +90,6 @@
       (let ((out (get-output-stream-string s)))
         (is (search "┌" out) "2x2 box still has borders")))))
 
-;; ── Text and Span Tests ───────────────────────────────────────
-
 (test text-creates-with-defaults
   "A text created with no arguments has reasonable defaults"
   (let ((txt (make-text "")))