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%.

org/dirty.org

@@ -40,8 +40,14 @@ inherit from this.
 
 * Tests
 
+** ~dirty-mixin-default-is-dirty~
+
+This test verifies that a freshly created ~dirty-mixin~ instance starts
+with ~dirty~ set to ~t~. This is the core invariant of the dirty tracking
+system — without this, the first render pass would skip new components,
+making them invisible until something explicitly marked them dirty.
+
 #+BEGIN_SRC lisp :tangle ../src/components/dirty-tests.lisp
-;; Dirty tracking tests are in box-tests.lisp (same test suite)
 (in-package :cl-tty-box-test)
 (in-suite box-suite)
 
@@ -49,12 +55,37 @@ inherit from this.
   "A dirty-mixin starts as dirty"
   (let ((c (make-instance 'dirty-mixin)))
     (is-true (dirty-p c) "new component should be dirty")))
+#+END_SRC
+
+** ~mark-clean-clears-dirty~
+
+This test checks that calling ~mark-clean~ on a dirty component sets its
+~dirty-p~ to ~nil~. This is called after a component is rendered,
+signaling that it is up-to-date and does not need re-render until the
+next change. Without this, every component would be re-rendered every
+frame.
+
+#+BEGIN_SRC lisp :tangle ../src/components/dirty-tests.lisp
+(in-package :cl-tty-box-test)
+(in-suite box-suite)
 
 (test mark-clean-clears-dirty
   "mark-clean sets dirty to nil"
   (let ((c (make-instance 'dirty-mixin)))
     (mark-clean c)
     (is-false (dirty-p c) "after mark-clean, should not be dirty")))
+#+END_SRC
+
+** ~mark-dirty-sets-dirty~
+
+This test verifies that a component that has been cleaned can be
+re-marked as dirty via ~mark-dirty~. This exercises the full lifecycle:
+new (dirty) → render (mark-clean) → state change (mark-dirty) → render
+again. It ensures the dirty flag is not a one-shot toggle.
+
+#+BEGIN_SRC lisp :tangle ../src/components/dirty-tests.lisp
+(in-package :cl-tty-box-test)
+(in-suite box-suite)
 
 (test mark-dirty-sets-dirty
   "mark-dirty sets dirty to t"