Add the card shape that nests another bundle's structure

Problem/Motivation

Half of what the forks call structure builders is one thing: render a referenced entity as a small card according to that bundle's own structure. Doing it as a shape is what removes the dozens of hand-written card builders and what makes a change to a bundle's teaser reach every place it is embedded.

Proposed resolution

• A card shape that takes a structure ID in its settings and renders the referenced entity through the same engine that renders a top-level response.
• Depth limit and cycle protection, driven by the cycles the scanner recorded — a self-referencing bundle must produce a bounded response, not a fatal.
• Access checking per referenced entity, so an unpublished target does not leak through a card.
• Cache metadata of every rendered target propagated to the response.
• Kernel tests including a cycle, an unpublished target and a target of a bundle with no structure.

Remaining tasks

Everything.