Add the entity detail resource base class

Problem/Motivation

The single most common endpoint — one entity of one bundle, rendered in full — is written by hand once per bundle per site today. In the target architecture it is a base class holding all the behaviour, with a generated subclass that declares three things: the structure, the breadcrumb trail and the counter field.

Proposed resolution

• MyrestEntityDetailResourceBase: load by id or by alias, check access, resolve the translation, build through the structure, attach cache metadata, wrap in the envelope, handle the not-found and access-denied cases through the existing error envelope.
• Declarative hooks for the subclass — structure id, breadcrumb trail, counter field — and nothing else; no branch may live in a subclass.
• Works for any content entity type.
• Functional coverage against a fixture bundle, including a missing entity, an unpublished one and a translation that does not exist.

Remaining tasks

Everything.