Refactor type checker for series and overloads

Introduce `unify_matched_overload` to handle type variable propagation
for overloaded functions. This ensures that when an overload is chosen,
its concrete parameter types are unified with the actual argument types.
This is crucial for resolving type variables nested within closures
called through overloaded functions.

Additionally, this commit:
- Allows `series` to infer schema type from `push` calls, simplifying
  its signature.
- Adds a fallback to `ValueSeries` in `rtl::series` if schema injection
  fails.
- Updates `StaticType::TypeVar` to return `Any` when accessed,
  simplifying field access logic.
- Adjusts tests to reflect the simplified `series` signature.

src/ast/types.rs

@@ -578,6 +578,9 @@ impl StaticType {
                         }
                     }
                     StaticType::Any => return Some(StaticType::Any),
+                    // TypeVar is unresolved — field access returns Any for now.
+                    // The type is concretized when the lambda is specialized.
+                    StaticType::TypeVar(_) => return Some(StaticType::Any),
                     _ => {}
                 }
                 None