Upgrade to Go 1.27 and use new stdlib features

[dreikanter]

Summary

Upgrade the project to Go 1.27 and adopt new standard library features introduced in recent Go versions:

• Replace sort.Strings() with slices.Sorted(maps.Keys()) for deterministic map iteration
• Use strings.Cut() instead of strings.Index() for cleaner string splitting
• Use strings.SplitSeq() and range-over-func for iteration instead of collecting slices
• Use errors.AsType() for cleaner type assertion in error handling
• Use min() builtin instead of manual comparison
• Update Go version requirement in go.mod from 1.25.8 to 1.27.0
• Update README to reflect new Go 1.27+ requirement

Checklist

• Updated CHANGELOG.md under Unreleased