Restore stashed WIP: live streaming plan, forms, CI pipeline, streaming demo

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

shared/sx/types.py

@@ -243,8 +243,24 @@ class PageDef:
     menu_expr: Any
     stream: bool = False        # enable streaming response
     fallback_expr: Any = None   # fallback content while streaming
+    shell_expr: Any = None      # immediate shell content (wraps suspense)
     closure: dict[str, Any] = field(default_factory=dict)
 
+    _FIELD_MAP = {
+        "name": "name", "path": "path", "auth": "auth",
+        "layout": "layout", "cache": "cache",
+        "data": "data_expr", "content": "content_expr",
+        "filter": "filter_expr", "aside": "aside_expr",
+        "menu": "menu_expr", "stream": "stream",
+        "fallback": "fallback_expr", "shell": "shell_expr",
+    }
+
+    def get(self, key, default=None):
+        attr = self._FIELD_MAP.get(key)
+        if attr is not None:
+            return getattr(self, attr)
+        return default
+
     def __repr__(self):
         return f"<page:{self.name} path={self.path!r}>"