Community widget packs

This directory is the reserved home for community packs. It is currently empty scaffolding: no pack has been contributed yet.

Community packs follow the same shape as the built-in ones (registry() over xtop-widget-api), and the kernel engine integrates them at compile time — there is no runtime pack loading:

1. Write the pack crate with a pub fn registry() -> HashMap<&'static str, WidgetRenderer>; packs that live here are added as members of this workspace (under custom/).
2. The kernel depends on the crate behind an optional Cargo feature and appends a Pack { name, renderers } entry to its precedence list (xtop/src/ui/layout/engine.rs::packs()).
3. Users select the pack with style.pack (global) or style.widgets.<name>.pack (per widget); missing names fall back to the base pack.

See docs/authoring.md for the full authoring contract. Runtime dynamic discovery of packs is deliberately deferred (workspace ROADMAP, §7).