Simplify plot weights

Summary

Reworks the Python weight-visualisation tooling: the old monolithic plot_weights.py (with its embedded grid_utils.py) is replaced by two focused tools and a shared utility module.

New tools

Tool	Purpose
yac_plot_weights.py	Interactive map showing interpolation weight arrows, core/halo grids, fixed-value target points, and hover tooltips
yac_plot_grid.py	Interactive map showing one or more YAC debug grid files with auto-discovery of all contained grids

Both tools support uv run for zero-friction dependency management.

Key improvements over plot_weights.py

• Debug-grid based — reads YAC's native debug grid format (.clo/.cla/.gid) so global indices always match the weight file exactly
• Core/halo distinction — halo cells are drawn dimmed and dashed when a core mask (.cmk/.vcmk) is present in the grid file
• Fixed-value target points — target cells assigned a constant by the fixed fallback interpolation method are shown as scatter markers with a legend
• Hover tooltips — hovering over a weight arrow shows src id, tgt id, and the weight value
• Shared _utils/plotting.py — projection setup, geodesic extent calculation, edge plotting, and label toggling are factored out and reused by both tools
• Interactive label toggling — press s/t (weights tool) or A/B/… (grid tool) to toggle cell-ID labels per grid

yac_generate_weights.py updates

• Supports named grids via --src-grid-name / --tgt-grid-name (loads from multi-grid files via uxarray.open_multigrid)
• Interpolation stack entries are now full JSON values on the command line (a plain string for methods without parameters, a ["method", {...}] array for parameterised methods)
• Writes src_grid_name / dst_grid_name attributes to the output weight file so yac_plot_weights.py can locate the correct grids automatically
• Target grid mask loading switched to netCDF4 for consistency

Closes #155 (closed), #70 (closed)

Checklist

• All code changes are covered by unit tests
• Code coverage did not deteriorate
• Code follows the Style Guide
• Documentation updated (weight_files.dox, yac_generate_weights.dox, components.dox)
• Manual running of configure/build jobs was successful
• Version info updated (doc/src/version_info.dox, Unreleased section)