fix: copy_from_slice panic when k > number of indexed elements

[npiesco]

Bug

search_layer in hnsw_const.rs panics with a copy_from_slice length mismatch when the requested number of neighbors (k / ef) exceeds the number of elements in the index.

Root cause

Two sites in search_layer (lines 309 and 321) use:

let found = core::cmp::min(dest.len(), searcher.nearest.len());
dest.copy_from_slice(&searcher.nearest[..found]);

copy_from_slice requires src.len() == dest.len(). When found < dest.len() (fewer elements in the index than requested), the source slice is shorter than dest → panic.

Fix

dest[..found].copy_from_slice(&searcher.nearest[..found]);

Slices dest to match found, which is what the next line (return &mut dest[..found]) already expects.

MRE

See tests/simple.rs::nearest_k_greater_than_index_size — inserts 8 vectors, searches with k=100:

#[test]
fn nearest_k_greater_than_index_size() {
    let (hnsw, mut searcher, _) = test_hnsw(); // 8 vectors
    let mut neighbors = vec![Neighbor { index: !0, distance: !0 }; 100];
    let found = hnsw.nearest(&&[0.0, 0.0, 0.0, 1.0][..], 100, &mut searcher, &mut neighbors);
    assert_eq!(found.len(), 8);
}

Before: panics with copy_from_slice: source slice length (8) does not match destination slice length (100)
After: returns 8 results

This surfaces whenever a caller passes k larger than the index size, or when filtered searches reduce the effective candidate set below k.