feat: add extensible operators and golden flows

README.md

@@ -28,8 +28,9 @@ nodes by purpose:
 | --- | --- |
 | Load | Inline data, datasource |
 | Combine | Append rows, join tables |
-| Filter | Filter rows, remove duplicates |
-| Transform | Select columns, derive column, aggregate, sort, limit |
+| Filter | Column filter, typed-expression filter, remove duplicates |
+| Transform | Select, derive, typed expression, conversion, replacement, aggregate, sort, limit, reusable subflow |
+| Quality | Quality rules, keyed reconciliation |
 | Output | Preview output |
 
 Join nodes have explicit left and right ports. Append nodes accept two or more
@@ -53,6 +54,15 @@ statement into the canonical graph. The dialect supports projection, aliases,
 filters, grouping, aggregate functions, sorting, limits, `DISTINCT`, append,
 and one two-source equi-join. It rejects DDL, DML, arbitrary subqueries,
 arbitrary functions, file access, and unchecked pass-through execution.
+The typed-expression library is a separate allowlisted AST evaluator shared by
+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.
 
 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
@@ -89,6 +99,16 @@ permissions block the delivery before source access or output publication.
 Confidential and restricted events are not accepted through the direct
 ingress; those require Core's transactional event bridge.
 
+Reusable subflow nodes pin a template reference, version, graph snapshot, and
+parameter values. Their single input is bound to an explicitly marked inline
+source inside the snapshot, parameter substitution is data-only, and nesting
+is bounded. This keeps completed run definitions reproducible even when the
+source template changes later.
+
+The executable fixtures in `fixtures/golden` cover the monthly structured-file
+reconciliation story and the sanctions-screening story with reviewable sample
+inputs and exact expected outputs.
+
 ## Development
 
 ```bash