Can Lucene support selective preloading of just slices/files within a CFS file?

### Description

At Amazon Product Search, we warm up our service by preloading vector files into RAM and use the `mmapDir.setPreload` API for this.
However, when vector files get compacted into a `.cfs` file, they do not get paged into RAM due to `setPreload` being called on the basis of `.vec`, `.veq` extensions.

This would also help for lexical search files that get compacted into `.cfs` files and which need preloading for service warm up purposes..

Start by reading the mmapDir.setPreload API and the CFS behavior described in the issue; trace how preload selection currently matches .vec and .veq files. Done means callers can selectively preload slices or files inside a .cfs file, covering both vector and lexical search files, with regression tests for the supported selection behavior.

Written by the indexing model from the issue text.