Fix release-readiness gaps before 2.0 - #23

Summary

Tightens the package before the 2.0 release by fixing four release-readiness gaps found in the audit.

What changed

• preserve repeated operations instead of overwriting earlier steps when the same operation type is chained multiple times
• execute repeated serialized operations in insertion order during processing
• preserve variant url values when rebuilding collections from serialized arrays
• append the target extension for converted variants when the source filename/path has no extension
• install imagick in CI so the Imagick-specific tests run instead of being skipped
• document the repeated-operation serialization shape in the README, processing docs, and changelog

Release impact

This keeps the existing single-operation serialization shape intact, while extending the on-disk format for repeated operations to an ordered list only when the same operation appears more than once.