Function render_context_prefix 

pub fn render_context_prefix(
    occurred_at: Timestamp,
    source_kind: &str,
    mentions: &[String],
    community_label: Option<&str>,
) -> String

Build a deterministic context prefix for a chunk. All fields come from projection rows; no model call is involved.

Format: [<occurred_at> · <source> · mentions: <e1>, <e2> · community: <label>]