Function stream_frame_from_expr 

pub fn stream_frame_from_expr(
    cx: &mut Cx,
    codec: Symbol,
    kind: FrameKind,
    expr: &Expr,
    envelope: FrameEnvelope,
) -> Result<ServerFrame>

Builds a stream boundary or chunk frame from an expression payload.

New stream-producing code should use this helper for StreamStart and StreamChunk frames so envelope handling stays consistent across server, agent, and stream-fabric adapters.