Playground demo: pre-apply diff viewer for a proposed file edit.

Exercises Raxol.UI.Components.Harness.DiffViewer's Pierre-style rendering with a realistic before/after edit -- syntax-highlighted Elixir, a long unchanged run (so hunk folding visibly kicks in with the default context: 3), a couple of lines removed, a couple added.

[m] cycles auto -> unified -> split. [w] toggles the simulated available width so :auto visibly flips between side-by-side (wide) and unified (narrow). [f] toggles folding: context: 3 <-> :all.