literate: convert org/modern-backend.org from doc-only to tangle source

Now tangles to modern.lisp + modern-tests.lisp.
Deleted hand-written originals and regenerated from org — GREEN.

src/backend/modern-tests.lisp

@@ -82,7 +82,7 @@
 (test osc8-escape
   "OSC 8 hyperlink escape wraps text"
   (is (equal (cl-tty.backend::osc8-link "http://example.com" "click here")
-             (format nil "~C]8;;http://example.com~C\\click here~C]8;;~C\\"
+             (format nil "~C]8;;http://example.com~C\\\\click here~C]8;;~C\\\\"
                      #\Esc #\Esc #\Esc #\Esc))))
 
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