Blueprints should store the reasons zone dispositions were changed

[sunshowers]

In #5493, we realized that blueprint diffs weren't quite clear about why a disposition was changed. That's because blueprints themselves don't have any persistent information about this.

To address this, we should consider storing a zone log (append-only?) next to each zone which indicates the particular blueprint an action was taken in, and why that was done.

For diffs it may be a bit confusing to see the zone log if the "before" blueprint isn't an ancestor of the "after" blueprint. We'll have to see how it looks, but may need to make blueprint diffs aware of the ancestor relationship, and enhance their output accordingly. Ancestor and cousin blueprint diffs definitely feel different to me in ways that implementing this will hopefully elaborate.

(We're reinventing bits of a source control system. :) )