Add comprehensive Bex workflow processing framework

Introduces a suite of classes to handle Bex-based workflows. This includes support for parsing, evaluating, and managing Bex bindings, expressions, and execution contexts, as well as utilities for metrics tracking and compute step execution.

Author: piotr-blue

src/main/java/blue/bex/compile/BexCompiler.java

@@ -185,8 +185,11 @@ private List<CompiledStatement> compileStatements(FrozenNode node, CompileScope
     }
 
     private CompiledStatement compileStatement(FrozenNode statement, CompileScope scope, String pointer) {
-        if (statement == null || statement.getProperties() == null) {
-            throw new BexException("Statement must be an operator object");
+        if (statement == null || statement.isEmptyNode()) {
+            return sourceStatement(currentFunction, pointer, "$return", new ReturnStatement(null));
+        }
+        if (statement.getProperties() == null) {
+            throw new BexException("Statement must be an operator object at " + pointer);
         }
         int count = 0;
         String op = null;
@@ -199,7 +202,7 @@ private CompiledStatement compileStatement(FrozenNode statement, CompileScope sc
             }
         }
         if (count != 1 || statement.getProperties().size() != 1) {
-            throw new BexException("Statement must have exactly one $ operator");
+            throw new BexException("Statement must have exactly one $ operator at " + pointer);
         }
         String bodyPointer = pointer + "/" + escape(op);
         CompiledStatement compiled;

src/main/java/blue/bex/value/BexNodeWriter.java

@@ -19,6 +19,9 @@ public static Node toNode(BexValue value) {
         if (value.isNull()) {
             return new Node();
         }
+        if (value instanceof NodeBexValue || value instanceof FrozenNodeBexValue) {
+            return value.toNode();
+        }
         if (value.isScalar()) {
             return value.toNode();
         }