wip: refactor into fewer crates

crates/engine/src/time/time_usec.rs

@@ -0,0 +1,15 @@
+use crate::*;
+
+/// Timestamp in microseconds
+#[derive(Debug, Default)] pub struct Microsecond(AtomicF64);
+
+impl_time_unit!(Microsecond);
+
+impl Microsecond {
+    #[inline] pub fn format_msu (&self) -> Arc<str> {
+        let usecs              = self.get() as usize;
+        let (seconds, msecs)   = (usecs / 1000000, usecs / 1000 % 1000);
+        let (minutes, seconds) = (seconds / 60, seconds % 60);
+        format!("{minutes}:{seconds:02}:{msecs:03}").into()
+    }
+}