feat: refac entity and impl Vamana.

[luoxiaojian]

This commit refactors the entity layer and introduces support for the Vamana index. Performance was evaluated on an ecs.r7.16xlarge instance using the Gist dataset, measuring single-threaded throughput (QPS, higher is better) under recall 0.9@10.

Version	Configuration	QPS
before	hnsw, m=16, efc=500, ef=141	1939.13
after	hnsw, m=16, efc=500, ef=144	1923.97
after	hnsw, m=16, efc=500, ef=144, contiguous	2554.07
after	hnsw, m=16, efc=500, ef=144, contiguous, batch_size=2	2761.00
after	vamana, alpha=1.5, max_degree=64, search_list_size=500, ef=50	2557.70
after	vamana, alpha=1.5, max_degree=64, search_list_size=500, ef=50, contiguous	3272.50
after	vamana, alpha=1.5, max_degree=64, search_list_size=500, ef=50, contiguous, batch_size=2	3562.73

Notes:

1. contiguous refers to a new entity layout that concatenates all chunks into a single contiguous memory block and enables transparent huge pages.
2. batch_size specifies the batch size during batched computation. The previous default batch size was 12.