Fix lambda multi-body, reactive island demos, and add React is Hypermedia essay

Lambda multi-body fix: sf-lambda used (nth args 1), dropping all but the first
body expression. Fixed to collect all body expressions and wrap in (begin ...).
This was foundational — every multi-expression lambda in every island silently
dropped expressions after the first.

Reactive islands: fix dom-parent marker timing (first effect run before marker
is in DOM), fix :key eager evaluation, fix error boundary scope isolation,
fix resource/suspense reactive cond tracking, fix inc not available as JS var.

New essay: "React is Hypermedia" — argues that reactive islands are hypermedia
controls whose behavior is specified in SX, not a departure from hypermedia.

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

shared/sx/ref/eval.sx

@@ -476,7 +476,10 @@
 (define sf-lambda
   (fn (args env)
     (let ((params-expr (first args))
-          (body        (nth args 1))
+          (body-exprs  (rest args))
+          (body        (if (= (len body-exprs) 1)
+                         (first body-exprs)
+                         (cons (make-symbol "begin") body-exprs)))
           (param-names (map (fn (p)
                               (if (= (type-of p) "symbol")
                                 (symbol-name p)