docs: the wallpaper engine and the burn

The appearance README gains the burn (the cover, the covered()
handshake and why, the shader cache trap) and the engine (sources,
shuffle, light run, homes, the draft page, where its state lives and
what starts it). The spec records what changed while building it: Prefs
instead of list files, a page of its own, folder sources, the header
buttons, the cover handshake.

AGENTS.md gains the Field.edited trap, which applies to every page with
a field and a button: a click does not end editing, so the value a
button acts on must be read on every change.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

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