fix(search): use total_cmp for NaN-safe float ordering (#667)

[mosuka]

Summary

Search, scoring, ranking, and segment-maintenance code ordered floats with partial_cmp(...).unwrap_or(Ordering::Equal) (and a few partial_cmp(...).unwrap()). That makes the comparator non-total: a NaN compares Equal to everything, which sort_unstable_by / BinaryHeap forbid (silent reorder, or a panic on recent std). The .unwrap() variants panic on NaN outright — e.g. deletion_ratio is 0/0 = NaN when total_docs == 0, inside a background merge sort.

This replaces all NaN-unsafe float comparators with f32::total_cmp / f64::total_cmp (a real IEEE-754 total order) — 70 sites across 22 files.

Approach

A balanced-paren transform rewrote RECEIVER.partial_cmp(ARG).unwrap_or(...Equal) and RECEIVER.partial_cmp(ARG).unwrap() to RECEIVER.total_cmp(ARG), never reordering ARG or the a/b operands, so every sort's direction and tie-break (.then_with(...), the geo lat→lon two-stage compare) is preserved structurally. total_cmp is an inherent method on f32/f64, so any non-float receiver would have failed to compile — all 70 receivers were float. Three now-unused Ordering imports were removed. bkd_tree.rs already used total_cmp as precedent.

For non-NaN values total_cmp matches partial_cmp exactly, so behaviour is unchanged except a NaN is now ordered deterministically instead of reordering or panicking. Public API is unchanged.

The HNSW Candidate (min-heap) / ResultCandidate (max-heap) Ord::cmp impls — the "float heaps" the issue names — are covered, along with the collector sort-key structs, the FieldValue::Float64 sort path, and the segment-maintenance priority / deletion_ratio sorts.

Tests

New nan_ordering_tests in hnsw/searcher.rs: pushing a NaN distance into the Candidate / ResultCandidate heaps must not panic, must not drop any element, and the finite distances must still pop nearest-first / furthest-first.

Verification

• cargo clippy -p laurus --all-targets -- -D warnings — clean
• cargo fmt --check — clean
• cargo test -p laurus --lib — 1110 passed (+2)
• cargo test -p laurus --tests (integration) — all 0 failed (no sort-order regression)

Out of scope (follow-up)

• Type-level enforcement via an OrderedFloat wrapper for float BinaryHeaps, to prevent the footgun from recurring.

Closes #667