Prep for Stream A of Phase 3 rust migration: f00's native-duct merge,
NEXRAD, and commercial-link enrichment move to Rust via the same
grid_tasks work queue. kind='analysis' (single row per run) vs
kind='forecast' (18 rows per run) lets one Rust worker dispatch on
payload type without a second table.
Forecast default keeps existing Rust claim_next behaviour unchanged
during cutover.
