Make geospatial_fabric_gpkg optional in config

[taddyb]

Context

After #173, the geodataset classes no longer need the GeoPackage for routing or training — flowpath attributes come from the adjacency zarr. The GeoPackage is only needed for:

1. Plotting — visualization notebooks use geometry to draw river networks
2. Legacy workflows — users who prefer the current gpkg-based setup

Currently, geospatial_fabric_gpkg is a required Path field in DataSources (validation/configs.py:47).

Proposal

Make geospatial_fabric_gpkg optional:

geospatial_fabric_gpkg: Path | None = Field(
    default=None,
    description="Path to the geospatial fabric geopackage (only required for plotting)"
)

Downstream changes

• Any code that accesses cfg.data_sources.geospatial_fabric_gpkg must handle None
• Plotting utilities should raise a clear error: "GeoPackage path not set. Provide geospatial_fabric_gpkg in your config to enable plotting."
• Config templates for routing should omit or comment out the gpkg field

Files to modify

• src/ddr/validation/configs.py — make field optional
• src/ddr/validation/plots.py — guard against missing gpkg
• config/templates/*.yaml — comment out gpkg for routing templates
• Example configs — remove gpkg requirement

Acceptance criteria

• ddr route works without geospatial_fabric_gpkg in config
• ddr train works without geospatial_fabric_gpkg in config
• Plotting code gives a clear error when gpkg is missing
• Pydantic validation passes with and without the field
• Existing tests pass

Depends on

• Refactor geodataset classes to read flowpath attrs from adjacency zarr #173