/// Master cycle counter. All timing in the emulator derives from this.
///
/// At 150 MHz, a u64 counter wraps after ~3,900 years.
///
/// The authoritative system-clock frequency lives in
/// [`crate::bus::Bus`]'s clock tree (see `bus/clocks.rs`). Callers who
/// need Hz should use `emu.bus.sys_clk_hz()`.