Content templates, part 3b: store exposed slot subtrees on individual entities

wim leers → credited effulgentsia → .

FYI this is inspired by what I proposed to @effulgentsia while discussing 📌 [PoC] Introduce a `ContentTypeTemplate` config entity Active , and which he encouraged to explore:

Been contemplating this for a couple of days.

I think what each item in a multi-cardinality XB field needs to look like is...a full tree. With a root UUID and everything.

The reason I need a root UUID is so that the content template, which assembles the final renderable tree, knows which part of the tree goes directly into the exposed slot, and which parts of the tree are just NestedArray::mergeDeep()'ed into the main tree (since it seems that the tree structure is never more than three levels deep).

In other words, given a single mini-tree loaded from an entity and targeting a particular slot in the template, the root of the mini-tree goes directly into the exposed slot, and the rest of the tree is just added to the main tree.

✨ Content templates, part 2: store a component tree in the template, and allow individual entities to fill in specific slots in that tree Active is in!

Okay -- needs test coverage (quite a bit of it, I'd think), and validation, but I've at least implemented my first stab at the render-time logic. Could use a quick review to confirm I'm not way off-base here.

Okay, this has a test now, so removing the tag. There are still some data integrity questions here, though, which surely means this is a stable blocker.