Add IndexElementFn for series type probing

Introduce `IndexElementFn` as a type alias for a function that maps a
`StaticType` to an optional `StaticType` representing its element type.
This allows the `TypeChecker` to determine the element type of indexable
types like `Series` without needing to be aware of their specific
implementations.

The `series_element_type` function is created and registered with the
`TypeChecker` to handle `Series` types. This design centralizes
type-specific logic for indexable types, making the `TypeChecker` more
generic and extensible for future indexable types such as `Stream`.

src/ast/compiler/call_hooks.rs

@@ -36,6 +36,16 @@ pub trait TypeInferenceAccess {
     fn try_record_promote(&self, a: &StaticType, b: &StaticType) -> Option<StaticType>;
 }
 
+/// Maps a concrete type to its element type for index-constraint propagation.
+/// Called by `bind_var` when a TypeVar with an index constraint is bound.
+///
+/// Returns `Some(elem_ty)` if `ty` supports integer-index access (e.g. `Series`),
+/// or `None` for all non-indexable types.
+///
+/// Register the appropriate implementation via `TypeChecker::new`. Each indexable
+/// type (Series, and in future Stream) contributes a match arm here.
+pub type IndexElementFn = fn(&StaticType) -> Option<StaticType>;
+
 /// Called after the call's return type is resolved.
 /// May replace the return type and/or perform unification side-effects (e.g. push).
 pub type PostCallHook = fn(