Async IO in streaming render — staggered resolve with io-sleep

Server (sx_server.ml):
- eval_with_io: CEK evaluator with IO suspension handling (io-sleep, import)
- io-sleep platform primitive: raises CekPerformRequest, resolved by eval_with_io
- Streaming render uses eval_with_io for data + content evaluation
- Data items with "delay" field sleep before resolving (async streaming)
- Removed hardcoded streaming-demo-data — application logic belongs in .sx

Application (streaming-demo.sx):
- streaming-demo-data defined in SX: 3 items with 1s/3s/5s delays
- Each item has delay, stream-id, and display data fields
- Shell renders instantly, slots fill progressively as IO completes

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

sx/sx/streaming-demo.sx

@@ -91,3 +91,48 @@
         (li (code ":content") " expression is re-evaluated with each yield's bindings")
         (li "Headers stream concurrently — independent of the data generator")
         (li "Future: SSE/WebSocket for re-resolving slots after initial page load")))))
+
+(define
+  streaming-demo-data
+  (fn
+    ()
+    (list
+      (dict
+        "stream-id"
+        "stream-fast"
+        "delay"
+        1000
+        "stream_label"
+        "Fast source"
+        "stream_color"
+        "emerald"
+        "stream_message"
+        "Resolved in ~1s (async IO)"
+        "stream_time"
+        "~1s")
+      (dict
+        "stream-id"
+        "stream-medium"
+        "delay"
+        3000
+        "stream_label"
+        "Medium source"
+        "stream_color"
+        "amber"
+        "stream_message"
+        "Resolved in ~3s (async IO)"
+        "stream_time"
+        "~3s")
+      (dict
+        "stream-id"
+        "stream-slow"
+        "delay"
+        5000
+        "stream_label"
+        "Slow source"
+        "stream_color"
+        "violet"
+        "stream_message"
+        "Resolved in ~5s (async IO)"
+        "stream_time"
+        "~5s"))))