Add optional YAC remapping backend with NNN/conservative support and CI coverage

[rajeeja]

Closes #1441, Close #897

• Added a YAC-backed remapping backend so UXarray can use YAC’s interpolation machinery when requested, while remaining entirely unchanged for existing UXarray workflows. The UXarray remap API now accepts backend="yac" plus yac_method and yac_options. If YAC isn’t available, throw error.
• Implemented YAC NNN (nearest‑neighbor) remap using YAC’s InterpolationStack.add_nnn, mapping UXarray grid locations (nodes/edges/faces) to YAC cloud grids (point-based) and remapping via YAC’s coupling API.
• Implemented YAC conservative remap for face‑centered data only, using YAC unstructured grids with face‑node connectivity. This preserves the correct conservative semantics for cell‑centered fields.
• Built a YAC-to-UXarray conversion path that:
  • Converts node coordinates to radians for YAC,
  • Constructs YAC UnstructuredGrid from UXarray face_node_connectivity,
  • Uses YAC points at the appropriate locations (CELL for conservative, CORNER for cloud).
• Added a backend parity test to compare UXarray NN and YAC NNN on a tiny deterministic 3‑node grid. This avoids tie‑breaking ambiguity and provides a direct sanity check that YAC’s nearest‑neighbor agrees with UXarray’s own implementation.