Prevent periodic halo double-counting in conservative spherical polygon interpolation (H32 NodeColumns → H64)

[Copilot]

Large source halos in conservative-spherical-polygon interpolation could include geographically identical periodic halo polygons in addition to owned source polygons, producing inflated target values near the periodic boundary (notably for H32 NodeColumns remapped to H64). This change removes that duplicate-path contribution and adds a focused regression for constant-field preservation in the affected configuration.

• Source KD-tree deduplication path (NodeColumns)

  • Updated source polygon candidate selection in ConservativeSphericalPolygonInterpolation::build_source_kdtree(...).
  • For node-based source data, halo classification now uses node_halo || node_ghost when deciding whether a node should go through remote (partition, remote_index) deduplication.
  • This aligns node-based behavior with cell-based halo handling and prevents periodic halo duplicates from being inserted as distinct source contributors.

• Regression coverage for the reported case

  • Added regression_halo_periodic_source_nodecolumns in test_interpolation_conservative.cc.
  • Targets Grid("H32") -> Grid("H64") with src_cell_data=false, tgt_cell_data=true.
  • Verifies non-halo target cells preserve constant input values across multiple constants (0.0, -2.5, 1.0), guarding against periodic-boundary amplification regressions.

• Representative change

  // NodeColumns source: treat halo nodes like ghosts for deduplicated source selection
  if (consider_src(node_part(inode), node_ridx(inode), node_halo(inode) || node_ghost(inode))) {
      ...
  }

💣💥☠️ Static Analyzer Report ☠️💥💣
https://sites.ecmwf.int/docs/atlas/static-analyzer/PR-364