docs: add a diagram to the fan-out ingestion explanation

[dimitri-yatsenko]

src/explanation/fan-out-ingestion.md explained the pattern in prose with no figure. Fan-out is the one shape in the model where a picture earns its place, because readers already know how to read a dependency graph and the whole point here is a write the dependency graph does not contain.

Why it is hand-built

Every other diagram in these docs is dj.Diagram output, and this one is not. That needs justifying, so the generator script carries the reason: dj.Diagram cannot draw this figure. Pointed at the page's own schema it renders Subject, Session, and Recording as three unconnected nodes, because there is no foreign key for it to follow. The absence is the subject.

So the notation is borrowed rather than emitted, and the script reads its values from what dj.Diagram itself emits (datajoint-python #1544) rather than inventing any:

• Tier by shape and color — Manual a green rounded box, Imported a blue ellipse.
• Dependency edges navy, no arrowheads; direction follows the layout.
• Edge weight carries cardinality.
• An underlined name introduces a primary-key attribute of its own.

Two of those do real teaching work in this figure. RecordingFile → Ingest is thick because Ingest declares only -> RecordingFile, so that key covers its entire primary key. And Ingest is the only un-underlined name, because it introduces no key attribute of its own — which is exactly why it can be an ingestion step without distorting the domain model.

The one deliberate departure

Fan-out writes are drawn dashed and with arrowheads. A genuine dependency edge has no arrowhead, so the arrowhead is the tell that this is not one. I avoided the bronze reserved for renamed foreign keys — these are not foreign keys at all.

Tables and the source_file attribute match the code sample directly above the figure, so the two read against each other.

Notes

• python scripts/gen_fanout_diagram.py regenerates it. No database and no graphviz needed, unlike gen_pipeline_diagrams.py.
• Long descriptive alt text, following the convention in normalization.md and relational-workflow-model.md.
• Dark palette follows prefers-color-scheme, consistent with the other committed figures — which means it tracks the reader's OS rather than the site's own light/dark toggle, since an <img>-embedded SVG cannot see the page's data-md-color-scheme. Pre-existing across all the diagrams here; noted in the script, not solved in this PR.