Add deftype and defeffect to SX type system (Phases 6-7)

Phase 6 — deftype: named type aliases, unions, records, and parameterized
types. Type definitions stored as plain dicts in *type-registry*. Includes
resolve-type for named type resolution, substitute-type-vars for
parameterized instantiation, subtype-resolved? for structural record
subtyping, and infer-type extension for record field type inference via get.

Phase 7 — defeffect: static effect annotations. Effects stored in
*effect-registry* and *effect-annotations*. Supports :effects keyword on
defcomp and define. Gradual: unannotated = all effects, empty list = pure.
check-body-walk validates effect containment at call sites.

Standard types defined: (maybe a), type-def, diagnostic, prim-param-sig.
Standard effects declared: io, mutation, render.

84/84 type system tests pass. Both Python and JS bootstrappers succeed.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

shared/sx/ref/special-forms.sx

@@ -209,6 +209,29 @@
   :example "(defmacro unless (condition &rest body)
   `(when (not ~condition) ~@body))")
 
+(define-special-form "deftype"
+  :syntax (deftype name body)
+  :doc "Define a named type. The name can be a simple symbol for type aliases
+        and records, or a list (name param ...) for parameterized types.
+        Body is a type expression: a symbol (alias), (union t1 t2 ...) for
+        union types, or {:field1 type1 :field2 type2} for record types.
+        Type definitions are metadata for the type checker with no runtime cost."
+  :tail-position "none"
+  :example "(deftype price number)
+(deftype card-props {:title string :price number})
+(deftype (maybe a) (union a nil))")
+
+(define-special-form "defeffect"
+  :syntax (defeffect name)
+  :doc "Declare a named effect. Effects annotate functions and components
+        to track side effects. A pure function (:effects [pure]) cannot
+        call IO functions. Unannotated functions are assumed to have all
+        effects. Effect checking is gradual — annotations opt in."
+  :tail-position "none"
+  :example "(defeffect io)
+(defeffect async)
+(define add :effects [pure] (fn (a b) (+ a b)))")
+
 
 ;; --------------------------------------------------------------------------
 ;; Sequencing and threading