Rename position delete index to clarify referenced data file key #3359

Is your feature request related to a problem or challenge?

PopulatedDeleteFileIndex::pos_deletes_by_path is keyed by the referenced data file's path. The current name leaves it unclear whether the key is the delete file's own path or the data file it applies to.

This tracks the naming follow-up requested in the #3035 review. In the September 21 review of #3145, mbutrovich explicitly asked for a separate tracking issue. The validation work in #3145 can remain separate.

Describe the solution you'd like

Rename the private field and matching local bindings from pos_deletes_by_path to pos_deletes_by_referenced_data_file in crates/iceberg/src/delete_file_index.rs, updating references and comments consistently. Preserve delete-file applicability, indexing, and scan behavior.

Validation should include the existing delete-file index tests, formatting, and Clippy. This is an internal naming cleanup with no intended public API or behavior change.

Willingness to contribute

I would be willing to contribute to this feature with guidance from the Iceberg Rust community.

Prepared with AI assistance; this issue records the requested follow-up and does not claim an implementation.