Refactor large Rust files in hk-core for long-term maintainability

[Orchardxyz]

I noticed that there are several Rust files in hk-core that are already over 1000 lines.

Based on the structure commonly seen in many well-maintained open source projects, keeping a single source file within a reasonable size usually makes the codebase easier to navigate, review, and maintain over time.

As a first step, I tried splitting rules.rs into smaller modules for better long-term maintainability. This change is intended as a structural refactor only, without changing behavior.(#92)

There are still several other large modules in hk-core that have not been adjusted yet. If you agree with this direction, we can continue optimizing the remaining large files in follow-up commits.

[RealZST]

Thanks for the refactor! Merged #92. The split definitely improves clarity.

For the other large files, I'd love to take them case by case to see where splitting helps most.