fix: add importAsync() and rebuildHnswIndex() to fix recall() after import()

[luminexo]

Summary

Fixes #315

After calling engine.import(data), engine.recall() always returns [] even though memories are present in this.memories. The HNSW vector index is not rebuilt during import, and the brute-force fallback never triggers because empty results don't throw errors.

Changes

1. Add importAsync(data, merge) - Async version that calls import() + rebuildHnswIndex()
2. Add rebuildHnswIndex() - Re-inserts all memories into HNSW index
3. Fix recall() - Fall through to brute-force when HNSW returns empty results (not just on errors)

Root Cause

import() populates this.memories but never calls vectorDb.insert(). After import, recall() calls vectorDb.search(), receives empty array, and returns it immediately. The catch block only triggers on thrown errors, not on empty results.

Usage

Test Plan

• Import data with importAsync() and verify recall() returns results
• Import data with import() + rebuildHnswIndex() and verify recall() returns results
• Verify brute-force fallback works when HNSW is empty

Fixes: #315