Type a projection

An Arrow schema in the JSON form of the Rust arrow-schema crate: fields and metadata, where each field has name, data_type, nullable, dict_id, dict_is_ordered and metadata. Bind results, projected scan plans and execution outputs can attach a versioned JSON plan description under the reserved metadata key spiral.execution.plan. This describes execution rather than row types. A missing executionId denotes a preview; an execution reports its actual physicalId and executionId. Literal payloads are omitted. Evaluate input schemas may opt into aggregate profiling with spiral.execution.options metadata (profile, optional opaque caller and partition). Projected scans use execution options independently of collectMetrics. Execution descriptions then include a profile keyed by physical node IDs. Fixed-schema Arrow responses preserve updates in a trailing scandal.execution dictionary column tagged by spiral.execution.frame; adapters restore plan metadata and strip this framing before exposing row columns. Empty batches can carry terminal or partial reports.