feat(cosinepair): add lazy constructor that skips init()

[Mec-iS]

Summary

Add CosinePair::lazy(m, top_k) that precomputes row_norms but skips the
Theta(n²) init() scan. query_row_top_k and query work unchanged because
they recompute distances on the fly from samples, row_norms, and parameters.

Methods that depend on the precomputed distances / neighbours fields
(closest_pair, ordered_pairs, query_row, distances_from) will panic
on a lazy instance — documented in the method doc comment.

Motivation

arrowspace-rs#141 reports
that build_laplacian_matrix scales at O(n²·d) because CosinePair::with_top_k
triggers init(), which performs a full symmetric pair scan (n(n-1)/2 dot products).
The consumer (_build_adjacency) only ever calls query_row_top_k — it never reads
self.distances or self.neighbours. The entire init() cost is wasted work.

Changes

• New method: CosinePair::lazy(m, top_k) — same validation and row_norms
  precomputation as existing constructors, no init() call.
• Two new tests: one verifying query_row_top_k results match the non-lazy path
  and distances/neighbours are empty; one verifying the single-row rejection.

Testing

• cargo test --all-features — 73 passed, 0 failed
• cargo clippy --all-features -- -Drust-2018-idioms -Dwarnings — clean
• cargo fmt --all -- --check — clean