Regroup the library without stopping the window

Pressing Regroup on a real library did not come back. Clustering 1,813 faces is
the textbook agglomeration — compute every pairwise cosine, then repeatedly scan
all live group pairs, score each with average link, and merge the best — and the
scan is inside the loop. Each merge rescans every surviving pair, and each score
is recomputed from scratch over every cross pair. Some 1.6 million pair scores
per merge, some 700 merges to do.

Three changes, none of which alter the answer.

Only above-threshold pairs can ever matter. An average that reaches the
threshold must have at least one term at or above it, so two groups with no
qualifying pair between them can never merge — not now, and not after any
sequence of merges, since merging only adds terms. The new `neighbours` module
produces exactly that sparse list: 7,875 pairs rather than 1.6 million on the
reference library. It also means the n^2 matrix is never materialised, so memory
goes from O(n^2) to O(edges) — 2.5 GB to a few hundred KB at 25,000 faces.

Merges cannot cross components, so the connected components of that graph are
independent problems: four hundred small agglomerations instead of one large one.

Average link is additive — sum(A u B, C) = sum(A, C) + sum(B, C) — so a merged
group's scores follow by addition. Kept as running (sum, count) per adjacent
pair, a score costs one division instead of a nested loop, and a heap with lazy
invalidation replaces the rescan.

Measured on the reference library: 0.28s, release, for all 1,813 faces.

An exact ANN index was tried and removed, and neighbours.rs records why so it is
not rediscovered as a good idea. IVF with a triangle-inequality bound is exact
and prunes beautifully on synthetic clusters; on real embeddings it prunes
*nothing* — 946 of 946 cell pairs survive. Median pair angle is 88.5 degrees and
the merge threshold is 66.2, so the bound needs cells of radius under ~10
degrees, but two photographs of the same person sit 36-60 degrees apart. No
ball-based partition of a 512-d near-orthogonal space can be tight enough. So
the scan stayed exhaustive and got an unrolled dot product and its blocks spread
across cores instead.

Correctness is held by keeping the old implementation as an oracle: three tests
run both engines over the same population — plain, under co-occurrence and
anchor constraints, and with a size-weighted calibration — and assert the
clusters are identical. Determinism is asserted at a size where the threaded
path is in play.

62 tests pass.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

core/dr-face/src/lib.rs

@@ -39,6 +39,7 @@ pub mod detect;
 pub mod embed;
 pub mod embedding;
 pub mod naming;
+pub mod neighbours;
 
 pub use align::{warp, Aligned112, Similarity, ALIGNED_EDGE, ARCFACE_TEMPLATE};
 pub use calibrate::{Calibration, Pairs, ReliabilityBand};