Computing visual clusters was O(n·k·dim) per Lloyd iteration over the whole
library, single-threaded — several seconds on an 80k-image library on first
view. Find centroids on a deterministic, evenly-strided sample (<=3000
embeddings) and then assign every image to its nearest centroid in one parallel
rayon pass. Libraries at or below the sample cap are unchanged.
Replace the greedy farthest-point seeding (which seeds outliers, leaving the
dense core under-represented on a sample so one centroid absorbed tens of
thousands of generic images) with proper density-aware k-means++ D² seeding,
made deterministic via a small fixed-seed SplitMix64 PRNG. This keeps clusters
balanced on large libraries.
A CLUSTER_CACHE_VERSION is folded into the tag-cloud cache key so existing
caches computed by the old algorithm are invalidated and recomputed. The
clustering timing line and the cache-write failure now go through the `log`
facade (debug/warn) instead of eprintln.
LyAhn
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