Default the embedding projection to PaCMAP (PCA-init, seed 42); keep UMAP as an option (#188)

Per the deep-research recommendation: PaCMAP preserves both local and global
structure better and more robustly than UMAP at these dataset sizes. Rows are
L2-normalized to mirror the previous cosine metric, PCA-initialized, and the
random_state is fixed for reproducibility. UMAP stays available via --method
umap. Regenerated the community UMAP with the PaCMAP default (292 communities).

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

Author: realmarcin