Add the processed text and plain text shapes

Problem/Motivation

Long text goes through the text format filters and through the URL rewriting that turns site-relative links absolute against the front-end domain — the domain_app value, which is being moved into this module's own configuration. Plain strings sometimes need trimming to a summary length. Both are done today inside the helper, with the domain read from a foreign module.

Proposed resolution

• processed_text: run the field through its format, rewrite relative URLs against the configured application domain, and attach the cache metadata the filters produce — the part that is easiest to lose.
• text: the raw string, optionally trimmed with a configurable length and a word boundary.
• A field with a summary returns both parts, and the shape says which one is which rather than picking for the site.
• Kernel coverage of a format that adds cache tags, and of a missing domain configuration — which must degrade to relative URLs and not to a 500.

Remaining tasks

Everything.