AXIS: return an identity/no-weight plan for identical source and target meshes

## Context

Requested from the [CECE PR #85 review](https://github.com/ufs-community/CECE/pull/85#discussion_r3847338439). Linked CECE tracking issue: https://github.com/ufs-community/CECE/issues/103

CECE currently maintains its own identical-grid check and no-weight copy path. Add an AXIS-supported identity path when source and target meshes are equivalent so clients can avoid constructing interpolation weights or maintaining duplicate mesh-equivalence logic.

## Proposed outcome

AXIS recognizes equivalent source and target meshes and returns an explicit identity/no-weight plan or equivalent status that clients can apply as an exact copy.

## Acceptance criteria

- Define mesh equivalence using dimensions/topology, ordered coordinates, longitude convention, and a documented numerical tolerance.
- Return an explicit identity/no-weight plan or equivalent status.
- Apply the identity plan as an exact copy while preserving nonspatial layers.
- Reject same-size meshes with reordered or mismatched coordinates when identity is required.
- Document behavior when meshes are not identical and ordinary interpolation is requested.
- Document the client-facing API and expected MPI/domain-decomposition behavior.
- Add focused tests for equivalent, reordered, and mismatched meshes.

Start by reviewing AXIS's mesh and interpolation interfaces, using the CECE PR #85 review and tracking issue as context. Define and document the equivalence and identity-plan behavior from the acceptance criteria, then add focused tests for equivalent, reordered, and mismatched meshes, including exact-copy, nonspatial-layer, and MPI/domain-decomposition behavior.

Written by the indexing model from the issue text.