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>

sx/sx/nav-data.sx

@@ -91,7 +91,9 @@
   (dict :label "sx sucks" :href "/essays/sx-sucks"
         :summary "An honest accounting of everything wrong with SX and why you probably shouldn't use it.")
   (dict :label "Tools for Fools" :href "/essays/zero-tooling"
-        :summary "SX was built without a code editor. No IDE, no build tools, no linters, no bundlers. What zero-tooling web development looks like.")))
+        :summary "SX was built without a code editor. No IDE, no build tools, no linters, no bundlers. What zero-tooling web development looks like.")
+  (dict :label "React is Hypermedia" :href "/essays/react-is-hypermedia"
+        :summary "A React Island is a hypermedia control. Its behavior is specified in SX.")))
 
 (define philosophy-nav-items (list
   (dict :label "The SX Manifesto" :href "/philosophy/sx-manifesto"