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

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

shared/sx/evaluator.py

@@ -1003,6 +1003,7 @@ def _sf_defpage(expr: list, env: dict) -> PageDef:
         menu_expr=slots.get("menu"),
         stream=stream,
         fallback_expr=slots.get("fallback"),
+        shell_expr=slots.get("shell"),
         closure=dict(env),
     )
     env[f"page:{name_sym.name}"] = page