Phase 6: Streaming & Suspense — chunked HTML with suspense resolution

Server streams HTML shell with ~suspense placeholders immediately,
then sends resolution <script> chunks as async IO completes. Browser
renders loading skeletons instantly, replacing them with real content
as data arrives via __sxResolve().

- defpage :stream true opts pages into streaming response
- ~suspense component renders fallback with data-suspense attr
- resolve-suspense in boot.sx (spec) + bootstrapped to sx-browser.js
- __sxPending queue handles resolution before sx-browser.js loads
- execute_page_streaming() async generator with concurrent IO tasks
- Streaming demo page at /isomorphism/streaming with 1.5s simulated delay

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

shared/sx/templates/pages.sx

@@ -15,6 +15,20 @@
       (body :class "bg-stone-50 text-stone-900"
         children))))
 
+;; ---------------------------------------------------------------------------
+;; Suspense — streaming placeholder that renders fallback until resolved.
+;;
+;; Server-side: rendered in the initial streaming chunk with a fallback.
+;; Client-side: replaced when the server streams a resolution chunk via
+;;   <script>__sxResolve("id", "(resolved sx ...)")</script>
+;; ---------------------------------------------------------------------------
+
+(defcomp ~suspense (&key id fallback &rest children)
+  (div :id (str "sx-suspense-" id)
+       :data-suspense id
+       :style "display:contents"
+    (if children children fallback)))
+
 (defcomp ~error-page (&key title message image asset-url)
   (~base-shell :title title :asset-url asset-url
     (div :class "text-center p-8 max-w-lg mx-auto"