QUE-384: Show the evals primer in figures and bring its facts up to main

Adds 18 figures and redraws the existing diagrams at a readable size. The
running example is one real filing: KREF's Q1 2026 10-Q, page 6, whose
printed net income, cell id and XBRL fact appear across the figures.

Re-verified against main at 63826e3: pydantic-ai-slim is now locked at
2.47.0, so the install is pydantic-evals==2.47.0 (its API matches 2.21.0
on every call the document uses); ranking now runs inside retrieve() and
falls back silently; the answer model is a constant; WorkOS sign-in gates
the HTTP API; the authored gold's document keys match nothing ingested.