fix: NetCDF output metadata + CRS hook + sink ownership

[DankerMu]

Closes #69.

• Add CF units + standard_name for mesh coordinates (mesh_node_x/y, mesh_face_x/y).
• Add optional CRS_WKT support (writes crs variable with spatial_ref / crs_wkt, links via grid_mapping).
• Fix Conventions for river/lake files (CF-only; no UGRID claim).
• Warn when SCHEMA is set (currently ignored).
• Make print sinks lifetime/ownership explicit via std::shared_ptr.

[DankerMu]

PR Review (#69 NetCDF output metadata hardening)

• CI: baseline + NETCDF builds passed.
• CF/UGRID metadata: adds units + standard_name for mesh_node_x/y and mesh_face_x/y (fixes missing-units surprises).
• Conventions: keeps "CF-1.10 UGRID-1.0" for element mesh outputs, and uses "CF-1.10" for river/lake files (no UGRID over-claim).
• CRS hook: optional NCOUTPUT_CFG CRS_WKT writes a crs variable (spatial_ref / crs_wkt) and links via grid_mapping; warns when missing.
• SCHEMA: warns when set (explicitly ignored for now; avoids silent no-op).
• Sink ownership: Print_Ctrl now stores sinks via std::shared_ptr; NetCDF sinks/files use shared ownership to make lifetimes explicit and robust to refactors.

LGTM.