monotonic `now` should not care about rollover, except to describe maximum duration

In the review session there was a question about if now should handle overflow. I think more precisely it shouldn't be interpreted in a way that assumes monotonic starts at zero. The only important part is the max knowable duration and that depends on if it is interpreted as signed or not.

java's docs are pretty decent on this https://docs.oracle.com/javase/8/docs/api/java/lang/System.html#nanoTime-- note that it indicates the value as a signed number

The value returned represents nanoseconds since some fixed but arbitrary origin time (perhaps in the future, so values may be negative)

The payload names no source file, test, or entry point. Start with the linked Java System.nanoTime documentation and the existing review discussion; done means the intended rollover, signedness, and maximum-duration semantics are explicitly decided.