The streaming render matched `List items` but SX's `(list ...)` produces
`ListRef` (mutable list) in the OCaml runtime. Data items were rejected
with "returned list, expected dict or list" — 0 resolve chunks sent.
Fixed both streaming render and AJAX paths to handle ListRef.
Added sandbox test for streaming-demo-data return type validation.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>