Add a plugin type for paragraph types that compute something

Problem/Motivation

A minority of paragraph types do not render their fields — they compute. Real examples from the sites: one sums service prices, one collects anchors from its sibling paragraphs, one pulls in a block that belongs to another module. These cannot be structures, and pretending otherwise puts a branch back into the engine.

Proposed resolution

• A plugin type for paragraph handlers, discovered by paragraph type, taking precedence over the generated structure when present.
• The plugin gets the paragraph, its siblings and the host entity, since the anchor-collecting case needs all three.
• Ship none of the site-specific ones in the module: they belong to the sites, and this issue delivers the extension point plus one example in a test module.
• Document it as one of the extension points in the customization guide.

Remaining tasks

Everything.