Add reactive islands spec: signals.sx + defisland across all adapters

New spec file signals.sx defines the signal runtime: signal, computed,
effect, deref, reset!, swap!, batch, dispose, and island scope tracking.

eval.sx: defisland special form + island? type predicate in eval-call.
boundary.sx: signal primitive declarations (Tier 3).
render.sx: defisland in definition-form?.
adapter-dom.sx: render-dom-island with reactive context, reactive-text,
  reactive-attr, reactive-fragment, reactive-list helpers.
adapter-html.sx: render-html-island for SSR with data-sx-island/state.
adapter-sx.sx: island? handling in wire format serialization.
special-forms.sx: defisland declaration with docs and example.

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

shared/sx/ref/render.sx

@@ -73,8 +73,8 @@
 
 (define definition-form?
   (fn (name)
-    (or (= name "define") (= name "defcomp") (= name "defmacro")
-        (= name "defstyle") (= name "defhandler"))))
+    (or (= name "define") (= name "defcomp") (= name "defisland")
+        (= name "defmacro") (= name "defstyle") (= name "defhandler"))))
 
 
 (define parse-element-args