SYSMOAS_ — Time monotonicity in history

We shall codify an additional invariant that ensures Commit timestamps increase monotonically, i.e. part commits have a strictly older timestamp.

Reasoning:

• this would make comparing and ordering Commits easier; given two commits from the history of a branch, it would take a single step (just comparing timestamps) to determine which one came earlier, without having to crawl through a potentially long chain of commits.
• there is little extra burden brought by the introduction of this constraint, as it is already how most use cases would work, with Commits being created one by one and timestamps being naturally assigned at the time of Commit creation; this additional invariant would only really constraint edge cases, such as when a large volume of history is imported from an external format, and timestamps are synthetically assigned en masse