Add centerlines layer support

[iandees]

Summary

• Adds centerlines as a supported layer type in the ETL pipeline
• New CENTERLINES_SCHEMA = ['hash', 'id', 'name'] in conform.py
• SourceConfig maps centerlines layer to the new schema
• process_one.py accepts centerlines as a valid layer
• Geometry preserved as-is (no centroid conversion, same as parcels/buildings)
• README updated with supported layer types

Test plan

• Verify openaddr-process-one --layer centerlines processes a centerlines source
• Confirm LineString geometry is preserved in output

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