Feature: prebuilt finder pattern

### Describe what you are looking for

The problem I am facing with stringzilla even though I like the approach is that every sz_find id doing to much of preparation work which make it very hard to optimize for the projects like file search where I need to apply the same needle to many files one by one

especially this function
https://github.com/dmtrKovalenko/StringZilla/blob/main/include/stringzilla/find.h#L360-L360

my proposal is to implement a finder api that is going to allocate all the preparation work once for the needle and then we can execute those per every haystack

### Can you contribute to the implementation?

- [x] I can contribute

### Is your feature request specific to a certain interface?

It applies to everything

### Contact Details

_No response_

### Is there an existing issue for this?

- [x] I have searched the existing issues

### Code of Conduct

- [x] I agree to follow this project's Code of Conduct

Read include/stringzilla/find.h around the referenced line and trace how sz_find prepares a needle for each haystack. Define and validate a finder API that performs needle preparation once and can be reused across multiple haystacks, while preserving the existing search behavior.

Written by the indexing model from the issue text.