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/rtl/series/mod.rs

@@ -15,21 +15,6 @@ use crate::ast::types::{NativeFunction, NodeIdentity, Purity, Signature, SourceL
 // 4. Type-Inference Hooks (registered via RTL bootstrap, no name coupling)
 // ============================================================================
 
-/// Index-element hook for the `Series` type.
-///
-/// Returns the element type of a `Series(inner)`, or `None` for all other types.
-/// Registered with `TypeChecker` so that the core type-checker remains agnostic
-/// about which concrete types support integer-index access.
-///
-/// When `Stream` (or future indexable types) is added, extend this function
-/// with additional match arms rather than touching the type-checker.
-pub(crate) fn series_element_type(ty: &StaticType) -> Option<StaticType> {
-    match ty {
-        StaticType::Series(inner) => Some(*inner.clone()),
-        _ => None,
-    }
-}
-
 /// Post-call hook for `(series n)`.
 /// Replaces the `Series(Any)` return type with a fresh TypeVar so that
 /// subsequent `push` calls can unify the element type (HM inference).