Remove IndexElementFn and series_element_type

The `IndexElementFn` trait and its associated `series_element_type`
function have been removed. The `TypeChecker` now directly inspects the
`StaticType::Series` variant to extract the element type when
propagating index constraints. This simplifies the type-checking logic
by removing an unnecessary layer of indirection and adhering to the
project rule of keeping the AST simple and pushing complexity into the
system.

src/ast/compiler/call_hooks.rs

@@ -35,16 +35,6 @@ pub trait InferenceAccess {
     fn get_slot_type(&self, addr: Address<VirtualId>) -> 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(