Is there any unambiguous source which says it is a halacha to eat "meat" at a Seudat miztvah?
I am aware of the many opinions which says that meat is only optional. I'm looking for sources which say it is not
In Tshuvos V'hanhagos 3:294, R. Moshe Shternbuch writes that ideally a bris should have a seudah with meat because of the use of the word 'sasson' (in addition to simcha) which implies a more extravagant affair -- something which people will notice as being fancier, like a wedding seudah. So at least for a bris and wedding seudos mitzvah, it would seem ideal to serve meat.
Well we allow it during the Nine Days, when we otherwise wouldn't. But that could simply be "the meal is so joyous you can have meat if you want it", not "you must have it."
Someone asked Rabbi Herschel Welcher about meat for Purim Seudah. He said the meal should be whatever you consider a festive meal; for many of us, that means meat; but if you're a dairy person and eggplant parmigiana or whatnot is what does it for you (and let's say you'd do it every so often for shabbos too), then that's fine too.
With regards to yomtov, in Temple times there was an obligation to eat sacrificial meat; today there's some talk about whether the "meat and wine for men" as described by the Gemara as fulfillment of "yom tov joy" is an objective requirement, or merely an illustration for most people.
Although the biography of Rabbi Moshe Feinstein (prefacing Igros Moshe vol. 8) said he would make sure to eat one piece of meat at all yomtov meals; even if the meal was dairy, he would then have a small piece of meat afterwards. But again that's his personal practice ...