/// Clock source for the template's `datetime.today()`.
///
/// The default ([`SystemLocal`](Clock::SystemLocal)) uses the host's local
/// time. Use [`Fixed`](Clock::Fixed) (or [`Utc`](Clock::Utc)) for deterministic,
/// host-independent output.
///
/// Shared by both the PDF and HTML paths. It controls `datetime.today()`, which
/// is independent of the output format.