Refactor: Replace Object with StreamStorage for streams

The `Object` trait is too generic and has been causing confusion. This
commit introduces `StreamStorage` as a dedicated trait for reactive
stream types.

This change involves:

- Renaming `Object` to `StreamStorage` in relevant places.
- Updating the `Value::Object` enum variant to `Value::Stream`.
- Modifying how streams are handled in the compiler, VM, and tests to
  use the new `StreamStorage` trait.
- Adjusting example scripts and tests to reflect the change in type
  representation.
- The `StreamNode` struct now explicitly holds its `element_type`.

src/ast/compiler/specializer.rs

@@ -245,7 +245,7 @@ impl Specializer {
             // Only replace the callee if the compiled value is actually a function/object.
             // If it's a scalar (like 30 from folding), we DON'T fold here.
             // We keep the Call but update the callee to the specialized version if it's an object.
-            if let Value::Object(_) | Value::Function(_) = &compiled_val {
+            if let Value::Stream(_) | Value::Function(_) = &compiled_val {
                 let specialized_callee = self.make_constant_node(
                     compiled_val,
                     StaticType::Function(Box::new(Signature {