Add sfdp force-directed graph-layout view (--method sfdp) + published Graph layout page (#190)

Adds a third 2D-reduction method: a Graphviz-sfdp force-directed layout of the
mutual-kNN graph over the KG embeddings — a global-structure-first graph view —
alongside the PaCMAP default and the UMAP option. Self-contained (sklearn kNN +
the sfdp binary; no new Python deps). Publishes a 'Graph layout' page and links
it from the landing's embedding card.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: realmarcin