fix: tune coarse clustering to avoid single-cluster collapse
min_cluster_size=100 with min_samples=5 collapsed to 2 clusters on
the current UMAP projection. Switch to min_cluster_size=50 with
min_samples=10 which produces ~64 coarse clusters with recognizable
topics.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>