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/render.sx

@@ -74,7 +74,8 @@
 (define definition-form?
   (fn ((name :as string))
     (or (= name "define") (= name "defcomp") (= name "defisland")
-        (= name "defmacro") (= name "defstyle") (= name "defhandler"))))
+        (= name "defmacro") (= name "defstyle") (= name "defhandler")
+        (= name "deftype") (= name "defeffect"))))
 
 
 (define parse-element-args