Add SWOT satellite geometry support to DDR routing

[taddyb]

Summary

• Add SWOT satellite geometry pipeline: join EIV fits to MERIT COMIDs via MERIT-SWORD crosswalk to derive top_width and side_slope
• Update Muskingum-Cunge routing (mmc.py) to blend SWOT/Lynker observed geometry with derived values, handling partial NaN coverage
• Extend configs, data loaders, and tests to support the new SWOT geometry fields

Changes

• references/geo_io/build_swot_geometry.py (new): Pipeline to build SWOT geometry NetCDF from EIV fits + MERIT-SWORD crosswalk for North American PFAF regions
• src/ddr/routing/mmc.py: New _apply_data_override() for 3-case blending (MERIT-only, full Lynker, partial SWOT); refactored _get_trapezoid_velocity() to derive width/slope from hydraulic geometry power law with data override
• src/ddr/routing/torch_mc.py: Updated to pass new geometry tensors through the routing interface
• src/ddr/geodatazoo/merit.py: Extended MERIT data loader for SWOT geometry fields
• src/ddr/validation/configs.py: New config fields for SWOT geometry paths
• scripts/train.py: Minor update for new config fields
• tests/: Updated routing and config tests for new interface

Test plan

• Run tests/routing/test_mmc.py and tests/routing/test_utils.py
• Run tests/validation/test_configs.py
• Verify build_swot_geometry.py produces valid NetCDF with sample EIV/crosswalk data

🤖 Generated with Claude Code

[taddyb]

Closing since experiments didn't net a positive change