[ENH] Rename FragmentSeqNo to FragmentIdentifier.

[rescrv]

Description of changes

This doesn't change anything about the code (yet), but we're going to be
having a FragmentIdentifier::SeqNo and FragmentIdentifier::UUID.

Test plan

CI

Migration plan

There is no need to migrate. Tests confirm that data format is unchanged.

Observability plan

N/A

Documentation Changes

N/A

[github-actions]

Reviewer Checklist

Please leverage this checklist to ensure your code review is thorough before approving

Testing, Bugs, Errors, Logs, Documentation

• Can you think of any use case in which the code does not behave as intended? Have they been tested?
• Can you think of any inputs or external events that could break the code? Is user input validated and safe? Have they been tested?
• If appropriate, are there adequate property based tests?
• If appropriate, are there adequate unit tests?
• Should any logging, debugging, tracing information be added or removed?
• Are error messages user-friendly?
• Have all documentation changes needed been made?
• Have all non-obvious changes been commented?

System Compatibility

• Are there any potential impacts on other parts of the system or backward compatibility?
• Does this change intersect with any items on our roadmap, and if so, is there a plan for fitting them together?

Quality

• Is this code of a unexpectedly high quality (Readability, Modularity, Intuitiveness)

[propel-code-bot]

Rename FragmentSeqNo to FragmentIdentifier across WAL3

This PR refactors the WAL3 crate to replace the FragmentSeqNo type with the new FragmentIdentifier type, applying the rename across manifests, garbage collection, batching, writer logic, tests, and auxiliary tooling. The change is intended to be a mechanical rename in preparation for future extensions (FragmentIdentifier::SeqNo and FragmentIdentifier::UUID) and preserves existing serialization behavior and semantics.

Key Changes

• Introduce FragmentIdentifier tuple struct in rust/wal3/src/lib.rs and update associated helper functions, error types, and path utilities
• Update manifest, garbage collection, batch manager, writer, manifest manager, reader, and copy logic to use FragmentIdentifier instead of FragmentSeqNo
• Adjust integration tests, common test utilities, and tooling (e.g., wal3-construct-garbage-from-manifest) to reference the new identifier type

Affected Areas

• rust/wal3/src/lib.rs
• rust/wal3/src/manifest.rs
• rust/wal3/src/manifest_manager.rs
• rust/wal3/src/gc.rs
• rust/wal3/src/writer.rs
• rust/wal3/src/batch_manager.rs
• rust/wal3/tests/*
• rust/wal3/src/bin/wal3-construct-garbage-from-manifest.rs

This summary was automatically generated by @propel-code-bot