- Variables a library adds to a collection this file extends are now a library update of their own. `library::pending` counts published variables of an extended collection's library collection that have no copy yet; `library.updates` lists them by name without an `id` (no copy yet); `library.acceptUpdates` brings them in, inheriting. Before, they only arrived with the library's next other update. The old special case in `accept_updates` is gone (the pending list now carries them).
- Extending an extended collection: a library collection that is itself an extended collection (a brand theme extending a core theme, published) can be extended in another file; values flow down the chain as each file accepts its library's updates, and every level's overrides stay. This already worked because a library's extended collection arrives as a plain copy; it is now checked end to end and documented, and the `sync_extensions` shortcut note says why local chains aren't followed (extending takes library collections only).
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