Add typed Arrow execution backend boundary

README.md

@@ -59,10 +59,26 @@ expression filters and calculated columns. It supports literals, columns,
 arithmetic, comparisons, boolean logic, `CASE`, safe casts, and a bounded
 string/numeric function catalogue without Python evaluation or effectful SQL.
 
-Node definitions, validators, preview executors, and SQL renderers are
-registered independently in the operator registry. Adding a node no longer
-requires another branch in the preview or graph-to-SQL dispatch loop. Operators
-that cannot be represented by constrained SQL declare that explicitly.
+Node definitions, validators, schema propagators, preview executors, and SQL
+renderers are registered independently in the operator registry. Adding a node
+no longer requires another branch in the preview or graph-to-SQL dispatch loop.
+Operators that cannot be represented by constrained SQL declare that
+explicitly.
+
+Every executable graph also has a versioned typed IR. It preserves graph
+identity and layout while giving ports, schemas, expressions, parameters,
+lineage, diagnostics, semantic hashes, and physical results stable contracts.
+The backend boundary exchanges bounded typed columnar batches that can be
+serialized as Arrow IPC. The deterministic Python executor remains the default
+and fallback.
+
+Install `.[analytics]` to enable the analytical backend. It executes only SQL
+generated from validated graphs in a separate short-lived DuckDB process.
+Inputs and outputs cross the process boundary as Arrow IPC; external access,
+extension installation/loading, persistent secrets, configuration changes,
+temporary spill files, and multiple DuckDB threads are disabled. Wall-clock,
+row, byte, memory, file-descriptor, and backend-concurrency limits are applied.
+Callers can explicitly request `reference`, `duckdb`, or `auto` for a preview.
 
 Preview reads at most 250 rows per source and enforces time, intermediate-row,
 result-byte, graph-node, and response-row bounds. Saved previews record the