fix(ruvector-router-core): rebuild HNSW index from storage on open + fix result heap orientation

[CoolDude1969]

Problem

Two bugs causing search() to always return 0 results after restart:

1. VectorDB::new() never rebuilt the index from storage — a fresh empty
   HnswIndex was created every time, so all persisted vectors were
   invisible to search. Closes ruvector-router-core HNSW: search returns 0 results from inserted vectors at scale (recall@1 << 1.0) #430.

2. search_knn_internal used a min-heap for result — it must be a
   max-heap so peek() returns the furthest candidate for correct pruning.
   With a min-heap, valid candidates were silently discarded at scale.

Changes

• vector_db.rs: rebuild index from storage in VectorDB::new() using
  get_all_ids() + insert_batch(); fix total_vectors stat initial value
• index.rs: change result heap to BinaryHeap<Reverse> (max-heap);
  fix termination and replacement conditions
• index.rs: remove ef_construction.min(m*2) beam clamp in insert()

Before / After

Before: ruvector search ... → Found 0 results
After: 1. 1 (score: 0.0000)
2. 2 (score: 0.0061)