Function persist_user_line 

pub fn persist_user_line(
    broker: Option<&Broker>,
    block_id: &str,
    data: &[u8],
    filestore: Option<&Arc<FileStore>>,
    global_output_zone: Option<&str>,
)

Persist a user message the controller wrote to the agent’s stdin, so parseHistoryLines can reconstruct its user_message node on reopen.

A transcript record like any other (Phase 5a-3c): written to the block’s file and the global zone with positions, and published — so a pane reading the stream by line never sees a gap where this record sits — but marked echo: "stdin", because the pane already shows the message (its agent-message-accepted node) and must not add a second one. Before 5a-3c this write published nothing at all.