Pre-warm the county boundary relation without creating a project.

Called from the picker the moment a user soft-selects a county, so the Neon roundtrip (a materialized view over the branch tiger_county, or on a commons workspace a one-row table from the app database's) overlaps with prompt composition instead of blocking the subsequent project-create. Idempotent — the relation is workspace-scoped and deterministically named by county_fips; concurrent warms are serialized inside ensure_county_boundary_mv by an advisory lock and the second caller fast-paths on the existing relation.

Workspace RBAC mirrors create_sandbox_project: FIPS not in the user's available geographies → 403, FIPS not in the workspace's workspace_counties table → 404, FIPS not present in the tiger_county the boundary is built from → 404. No project row is created on any path — the warm flow is read-only at the app-DB level.