Update example configs for zero-configuration out-of-the-box routing

[taddyb]

Context

After #173, #174, and #176, all the pieces are in place for a zero-download routing experience. The example configs need to be updated to reflect the new data layout:

• Adjacency zarr (with flowpath attrs): in-repo
• Trained weights: in-repo (already there)
• KAN attributes: S3 (Lynker) / Zenodo (MERIT)
• Lateral inflows: S3 (lazy-loaded via xarray/icechunk)
• Observations: S3
• GeoPackage: not required

Proposal

Update example configs so that ddr route --config-name example works immediately after git clone + uv sync:

Lynker example config

mode: routing
geodataset: lynker_hydrofabric
device: cpu  # CPU by default for broadest compatibility

data_sources:
  attributes: s3://mhpi-spatial/hydrofabric_v2.2_attributes/
  conus_adjacency: ./../../data/hydrofabric_v2.2_conus_adjacency.zarr  # in-repo
  statistics: ./../../data/statistics
  streamflow: s3://mhpi-spatial/hydrofabric_v2.2_dhbv_retrospective
  # geospatial_fabric_gpkg omitted — not needed for routing
  # observations omitted — not needed for routing mode

experiment:
  checkpoint: ./ddr-v0.5.2.lynker_hydrofabric_trained_weights.pt
  batch_size: 7
  start_time: 1995/10/01
  end_time: 1995/10/10
  warmup: 3

Key changes

• Remove geospatial_fabric_gpkg (no longer required)
• Set device: cpu for out-of-the-box compatibility
• Keep S3 paths for attributes/streamflow/observations (lazy-loaded, no pre-download)
• Keep short time range for quick example (10 days)
• Add inline comments explaining each field

Config templates

Also update config/templates/ routing templates with the same pattern, plus clear comments explaining what each field does and what values are valid.

Files to modify

• examples/merit/example_config.yaml
• examples/lynker_hydrofabric/example_config.yaml
• config/templates/merit_routing.yaml
• config/templates/lynker_routing.yaml
• config/templates/merit_training.yaml (comment out gpkg, note it's optional)
• config/templates/lynker_training.yaml (same)

Acceptance criteria

• ddr route --config-name example works from a fresh clone with only uv sync
• No placeholder paths like /path/to/ remain in example configs
• Every config field has an inline comment
• Routing templates don't require gpkg
• Training templates note gpkg is optional

Depends on

• Make geospatial_fabric_gpkg optional in config #174
• Commit pre-built adjacency zarrs to repo for out-of-the-box routing #176