Move archival artifacts to dedicated directory

[bmeyers]

Currently, files marked as "archival only" live alongside active code:

• tests/fixtures/*.ipynb — 6 notebooks from the deprecated dataio module
• sdt_dask/examples/dev_scripts/rev_far_pvdb_dask.py — depends on removed PVDBPlug

These are documented with READMEs explaining they won't execute, but they still appear in IDE file trees and could confuse new contributors or be accidentally imported.

Proposed change:

• Create a top-level archival/ directory with subdirectories mirroring the original structure
• This is support future archiving needs for the project
• Move the 6 fixture notebooks to archival/tests/fixtures/
• Move rev_far_pvdb_dask.py to archival/sdt_dask/examples/dev_scripts/
• Keep the README.md files explaining what each artifact was and why it's non-functional
• Add an archival/README.md at the top level explaining the purpose of the directory
• Update any references (e.g., .gitignore, docs) if needed

Checklist:

• Create archival/ directory structure
• Move notebooks and script (use git mv to preserve history)
• Preserve existing README documentation in each subdirectory
• Add top-level archival/README.md
• Verify no active code imports from the moved files
• Update .gitignore if it references any moved paths