Add the paragraphs shape

Problem/Motivation

Paragraphs are the most divergent part of the whole codebase: 4 handlers on one site, 32 on another, 38 on the third, with only 9 types in common. The shape itself, though, is simple — a list of rendered paragraph structures — and separating it from the per-type handling is what makes the per-type work tractable at all.

Proposed resolution

• A paragraphs shape that iterates the referenced revisions and renders each through its type's structure, in field order.
• Nesting to arbitrary depth with a configurable bound, since paragraphs inside paragraphs are normal on two of the three sites.
• A paragraph type with no structure yet produces a fragment marked as unmapped and a queue entry, rather than being skipped silently.
• Per-type handling is out of scope here and is filed separately.

Remaining tasks

Everything.