Function export_state 

pub fn export_state(
    mapping: &mut SyncMapping,
    heddle_repo: &Repository,
    repo: &Repository,
    state_id: &StateId,
    options: ExportStateOptions<'_>,
) -> GitProjectionResult<Option<ObjectId>>

Export a single state to Git for audience.

Returns Ok(None) — absence — when the state’s effective visibility tier is not visible to audience: the public mirror never mints a Git commit (no stub, no partial tree) for an embargoed state (spike §5.0/§5.3). The caller realizes downward-closure by also withholding any state whose parent was withheld, so an embargoed commit and its descendants stay absent from the mirror.