Optimize ToString() performance via JIT bounds-check elimination and update documentation

[Seramis]

Description

This PR introduces a JIT optimization in the Fill<T> method to improve ToString() performance and synchronizes project documentation with recent benchmarks.

• JIT Optimization: Added a guard clause to Fill<T> that validates Span and map lengths upfront. This hints to the JIT compiler that subsequent indexed accesses are safe, effectively eliminating redundant runtime bounds-checking for the 26 character assignments.
• Documentation: Updated README.md and PACKAGE.md with the latest benchmark results and clearer feature definitions to better reflect the library's current features.

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update
• Other (please specify): Performance optimization

Checklist

• The PR is submitted to the correct branch (main).
• My code follows the project's coding style. (.editorconfig)
• I have commented my code, particularly in hard-to-understand areas and public interfaces.
• I have added or updated tests for the changes I made.
• All new and existing tests passed.
• I have updated the documentation where applicable.

Additional Notes

The Fill<T> method is a hot path for ULID string encoding. By ensuring span.Length is at least UlidStringLength and map.Length is at least 32 ($1 \ll 5$), the compiler can optimize the massive block of assignments into a linear sequence of instructions without branching for safety checks at every index.