[WIP] CAGRA ACE improvement: Global Rev-edge and exact initial kNN graph

[enp1s0]

This PR improves CAGRA ACE graph construction by:

• Adding an optional global cross-partition reverse-edge pass.
  • For higher recall
• Supporting exact brute-force kNN construction for partial graphs.
  • When the partition size is small, brute-force-search-based kNN build is faster

TODO

• Add test
• Add performance results