函数TOP_K，BOTTOM_K增加offset功能

目前TOP_K，BOTTOM_K参数k最多只能查询前1000名
希望增加offset参数，比如 `select top_k(s1, k=1000, offset=1000)` ，这样可以查询第1000到2000名

Start by locating the TOP_K and BOTTOM_K query functions and examining how their existing k limit is applied. Use the example query with offset=1000 as the target behavior, and verify that results can cover ranks 1000 through 2000 rather than only the first 1000.

Written by the indexing model from the issue text.