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/backend/detection.lisp

@@ -1,12 +1,8 @@
 (in-package :cl-tty.backend)
 
-;;; ─── Detection cache ────────────────────────────────────────────────────────
-
 (defvar *detected-backend* nil
   "Cached backend instance from detect-backend. Nil = not yet detected.")
 
-;;; ─── Environment probe ──────────────────────────────────────────────────────
-
 (defun detect-backend-by-env ()
   "Check COLORTERM environment variable for modern terminal support.
 Returns :modern if COLORTERM contains 'truecolor' or '24bit', nil otherwise."
@@ -16,15 +12,11 @@ Returns :modern if COLORTERM contains 'truecolor' or '24bit', nil otherwise."
                    (search "24bit" colorterm :test #'char-equal)))
       :modern)))
 
-;;; ─── TTY probe ──────────────────────────────────────────────────────────────
-
 (defun detect-backend-by-tty ()
   "Check if stdout is a real terminal (not a pipe/redirect).
 Returns T if stdout is interactive, nil otherwise."
   (interactive-stream-p *standard-output*))
 
-;;; ─── DA1 terminal query ─────────────────────────────────────────────────────
-
 (defun query-terminal (query &optional (timeout 0.1))
   "Send QUERY string to terminal and return any response received within
 TIMEOUT seconds. Returns the response string, or nil if no response."
@@ -41,14 +33,12 @@ TIMEOUT seconds. Returns the response string, or nil if no response."
 (defun detect-backend-by-da1 ()
   "Send DA1 (ESC[c) query and check for kitty terminal response code.
 Returns T if terminal reports kitty compatibility codes."
-  (let ((response (query-terminal (format nil "~C[c" #\Esc))))
+  (let ((response (query-terminal (format nil "~C[c" (code-char 27)))))
     (when response
       ;; DA1 response format: ESC [ ? digits ; digits c
       ;; Kitty reports code 62 in the response
       (search "?62" response))))
 
-;;; ─── Orchestrator ───────────────────────────────────────────────────────────
-
 (defun detect-backend ()
   "Auto-detect the appropriate backend for the current terminal.
 Returns a backend instance (modern-backend or simple-backend).