Or may be OP had issues all along and is just looking for an opportunity.
NO, this is really not it. We never had a problem, and because it started with a corner of the yard behind my kids' swing set, I hadn't realized how bad it was until they started fighting about who had to go into "the mud pit" to retrieve a lost ball. I've been hesitating because I felt like I couldn't "prove" it was the new build. I wasn't certain myself because of where the water was accumulating. But now I saw (and have video of) water gushing from the retaining wall and running straight across the yard to the corner/'mud pit.'
PP, I asked the drainage companies and our landscapers about regrading our lot (honestly it seemed like it could be less expensive than the massive pump station or rain gardens they were proposing) But they said it's not an option. Our lot is already graded on a very slight decline away from our house.
I recognize that we may still be SOL but I'm going to call. There's no fencing or anything of the kind around their lot and certainly no drainage mechanisms in place to funnel the water away from our property. The drainage companies we'd called said that sometimes that doesn't happen until they have actually finished the house, but given how long the construction is going on, it doesn't seem right that they can do absolutely nothing. Thanks for all the really good advice & expertise on this thread!