* fix(vamana): medoid entry point, concurrent build crash and prune quality
- Add DiskANN-standard medoid (centroid-closest point) as the persisted
entry point, computed at dump time, replacing the fixed node-0 start.
- Fix data races on node_chunks_ / dist_chunks_ / node_chunk_bases_ during
concurrent build: add mutexes with double-checked locking and pre-reserve
capacity so the lock-free read fast path never hits reallocation.
- Fix RobustPrune for metrics with signed internal distance (quantized int8
cosine): introduce IndexMetric::build_distance_offset() to shift distances
into a non-negative range, restoring a geometrically meaningful
occlude_factor and improving graph quality / low-ef recall.
Add modernize-use-override to .clang-tidy and apply fixes across
src/ and tests/: replace redundant virtual with override, annotate
missing override on derived methods, and drop redundant virtual on
already-overridden methods.
luoxiaojian
egolearner