Add layered zip indexes

[klauspost]

This will allow to specify multiple indexes and have them searchable as a single entity.

Indexes are added in order and the layered index will represent the combined state.
That means that on a file with a given path the newest version will be returned.

A generic comparable type can be provided as the index reference type, so for any given file this can be seen in the results.

An index can be added as a "delete" index. This means that any files in this will be removed from the underlying layers.

• Add LayeredIndex[T] for combining multiple zip indexes with generic references
• Implement file override semantics: newer layers override older ones
• Add delete layers to remove files from previous layers
• Automatic empty directory cleanup when files are deleted
• Binary search optimization: files sorted by name on layer addition
• Find operations use O(log n) binary search instead of O(n) linear search
• Concurrent serialization/deserialization with user-provided RefSerializer[T]
• Manual msgpack format: version + layers with ref data as binary arrays
• Comprehensive test suite with 15+ test scenarios and benchmarks
• Iterator support for Go 1.23 range syntax
• Complete API: layer management, file access, conversion, serialization
• Updated README with usage examples and API documentation