Cover the scanner with kernel tests on a fixture model

Problem/Motivation

The scanner cannot be tested against a real site — none of the three has an exported configuration, and a database dump is not a test fixture. Without tests it is the component whose bugs are hardest to notice, because a wrong map produces plausible-looking generated code.

Proposed resolution

• A test module that installs a deliberately varied model: several bundles, every field type of the shape table, cardinality 1 and unlimited, required and optional, references to nodes, terms, media and paragraphs, nested paragraphs, a hierarchical vocabulary, two languages.
• Kernel tests asserting the map for that model key by key, not just that it is non-empty.
• Cases that are easy to get wrong: a bundle with no fields, a field with no data, a reference with no target bundles restricted, a cycle.

Remaining tasks

Everything. The fixture model is reused by every later stage, so it is worth building properly here.