Add Stream support for field accessors

The type checker now correctly handles unwrapping `Stream(T)` types for
lambda arguments.
A new `build_map_stream` function has been added to `rtl/streams.rs` to
facilitate field access on streams.
The `create-random-ohlc` function now returns a `Stream` instead of a
`Series`.
Examples have been updated to reflect these changes and demonstrate
stream usage.

src/ast/rtl/streams.rs

@@ -416,6 +416,34 @@ pub fn build_pipeline_node(
     })
 }
 
+pub fn build_map_stream(input: Rc<dyn ObservableStream>, field: Keyword) -> StreamNode {
+    let executor: Box<PipeFn> = Box::new(move |args: &[Value]| -> Value {
+        let val = &args[0];
+        if let Value::Record(layout, values) = val
+            && let Some(idx) = layout.index_of(field)
+        {
+            return values[idx].clone();
+        }
+        Value::Void // In streams, Void acts as a filter
+    });
+
+    let pipe = Rc::new(RefCell::new(PipeStream::new(
+        format!("map:{}", field.name()),
+        1,
+        Some(executor),
+    )));
+
+    let adapter = Rc::new(RefCell::new(SourceAdapter {
+        target: pipe.clone(),
+        target_index: 0,
+    }));
+    input.add_observer(adapter);
+
+    StreamNode {
+        inner: pipe,
+    }
+}
+
 // ============================================================================
 // Script Integration (RTL Registration)
 // ============================================================================
@@ -439,7 +467,7 @@ pub fn register(env: &Environment) {
         "create-random-ohlc",
         StaticType::Function(Box::new(Signature {
             params: StaticType::Tuple(vec![StaticType::Int, StaticType::Int]),
-            ret: StaticType::Series(Box::new(StaticType::Record(ohlc_layout.clone()))),
+            ret: StaticType::Stream(Box::new(StaticType::Record(ohlc_layout.clone()))),
         })),
         Purity::Impure, // Modifies global generator registry
         move |args: &[Value]| {