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Five-task plan executing spec 0001: scalar set, Column/Window, KindMismatch, AnyColumn, then a workspace build/test/clippy + surface-purity gate. Each task adds its own module + re-export so the crate compiles green after every task; 14 unit tests pin the substrate contract incl. the wrong-kind-push guard. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>