WIP: Generalize Sparse Indices

[lroberts36]

PR Summary

Generalized sparse identifiers from a single integer to a typed SparseID, then used that type throughout sparse-field metadata, packs, and lookup paths. Added support for grouped sparse control so a set of controller fields can allocate and deallocate together, while keeping the legacy single-controller behavior and restart exactness intact for existing sparse-advection and restart workflows.

PR Checklist

• Code passes cpplint
• New features are documented.
• Adds a test for any bugs fixed. Adds tests for new features.
• Code is formatted
• Changes are summarized in CHANGELOG.md
• Change is breaking (API, behavior, ...)
  • Change is additionally added to CHANGELOG.md in the breaking section
  • PR is marked as breaking
  • Short summary API changes at the top of the PR (plus optionally with an automated update/fix script)
• CI has been triggered on Darwin for performance regression tests.
• Docs build
• Any contribution that was created or modified with the assistance of generative AI is disclosed here and in code following the guidelines
• (@lanl.gov employees) Update copyright on changed files