perf(vector/search): `Vec<Candidate>` heap allocation grows on graph traversal — pre-size to ef_search

[mosuka]

Round-3 perf push sub-issue (tracked under umbrella #536).

[S] Vec<Candidate> heap allocation grows on graph traversal — pre-size to ef_search

• Where: hnsw/searcher.rs:419-420 — BinaryHeap::new() × 2 with no with_capacity.
• Current behavior: Heaps grow geometrically on push, reallocating O(log N) times.
• Suggested direction: BinaryHeap::with_capacity(ef_search * 2) for the result heap;
  candidate heap can be sized similarly.
• Risk / scope: Trivial.

---

ID: VS-37 — see ~/.claude/tasks/laurus/20260523_perf_round3_audit/task_list.md for the full Round-3 issue list.