Pretty straightforward: I contracted for one year with a team and during that time I established a quality assurance framework that has been scaling up for several months. Now my contract is up and I'm changing jobs (I would have stayed but am moving to a new city and going back to school part time), and they're replacing me with two full-time people to take on the QA work. People tell me that demonstrates how essential I was to the team, but I have two questions:
Is it appropriate to include that fact on my resumé?
If so, is there a tactful way to say it that sounds matter-of-fact and not boastful?
Absolutely put this on your resume but not in the way that you have worded it.
You are not being "replaced by two people" but rather your former company is impressed with the framework you helped establish and have showed their approval by investing in additional resources to help grow the process. This does indeed show your work as valuable and therefore worthy of inclusion on your resume.
I would word this something like the following:
Led effort to establish quality assurance framework [name of software] within development lifecycle. As of departure, company had invested two additional FTEs to grow this system.
It is an odd thing to put on CV, and I wouldn't do it.
Not because it's inapropriate (is it?), but because you cannot be sure of the reason for such change. It feels completely normal that you need two people to replace someone who was establishing something because now they not only need to learn the thing you made, but also continue to provide the service you were providing.
In other words: being essential to one company isn't really a value to another.
As a hiring manager, adding to your resume that you were replaced by 2 people doesn't tell me that you were did the work of 2 people. It tells me that you have don't have something else better to put on your resume instead of that statement.
In my years of doing resume reviews, I have NEVER seen anyone write this and the idea of it is very off-putting to where I would likely reject your resume. Instead, tell me what you accomplished for the company and what impact you had in the time you were there.