Refactor: Simplify series creation and infer element type

The `series` constructor now only accepts the `lookback` limit. The
element type is inferred by the type checker and implicitly added as a
schema argument during compilation. This simplifies the API and
centralizes type inference.

This change also includes:
- Updates to example scripts (`HMA.myc`, `sma.myc`, `soa_series.myc`) to
  reflect the new `series` signature.
- Modifications to the `TypeChecker` to handle the inferred schema
  injection.
- An addition of a regression test for type inference through nested
  closures with `series`.
- Removal of `#[allow(dead_code)]` from several `TypeChecker` methods as
  they are now used.
- Update to `rtl/prelude.myc` macro `cache`.
- Update to `rtl/series/mod.rs` to reflect new signature.
- Update to `ast/types.rs` to allow `series` to be indexed with an
  integer to retrieve its element type.

src/ast/rtl/series/mod.rs

@@ -11,12 +11,15 @@ use crate::ast::types::{Purity, RecordLayout, Signature, StaticType, Value};
 // ============================================================================
 
 pub fn register(env: &Environment) {
-    // (series lookback_limit template_or_type_keyword) -> Series
+    // (series lookback_limit) -> Series
+    // The element type is inferred by the HM type checker from subsequent push calls.
+    // After type checking, the compiler elaborates (series n) into (series n schema)
+    // so the runtime always receives the explicit schema argument.
     env.register_native_fn(
         "series",
         StaticType::Function(Box::new(Signature {
-            params: StaticType::Tuple(vec![StaticType::Int, StaticType::Any]),
-            ret: StaticType::Any,
+            params: StaticType::Tuple(vec![StaticType::Int]),
+            ret: StaticType::Series(Box::new(StaticType::Any)),
         })),
         Purity::Impure,
         |args: &[Value]| {