Function build_receipt_event 

pub fn build_receipt_event(
    conn: &Connection,
    caller_user_id: i64,
    receipts: &[ReceiptRow],
) -> Result<Option<Value>, MatrixError>

One coalesced m.receipt ephemeral event (plan §3.6): every m.read receipt (visible to the whole room by definition) plus ONLY the caller’s own m.read.private receipt — Matrix’s own privacy rule that a private receipt must never reach another user’s /sync. None when receipts contained only OTHER users’ private receipts — after the privacy filter there is nothing left worth an (otherwise empty-content) event at all, and an empty-but-present m.receipt event would wrongly count as “this room changed” for the emptiness check that gates a room’s inclusion.