"Solo three week trip (~21 days). Zero resupply opportunity. Water only cooking. Long days of traveling necessitated by water source needs. Temperatures from over 100 deg F to 25 deg F.
Individually packaged meals or bulk?
Bulk, but maybe pack a couple of evening meals as individual for those days toward the end when you're going to be really tired and maybe don't want to screw around with measuring out stuff from bulk bags.
What goodies can withstand the heat? Waxed chocolates?
Hard candies, peanut brittle or toffee, brown sugar, maple sugar, chocolate if you bag it well(maybe vacuum seal?). Wait until morning after a 25 degree night to open it and it will be rock solid.
Need some variety and good fat/protein heavy sources.
Nuts, lots of varieties including Blue Diamond almonds which come in a variety of tasty flavors(Maui Onion, Chili Lime, Smokehouse Habanero, Honey Roasted, etc), Nido full fat dried milk(try mixing it up with some maple or brown sugar-yummmm), olive oil or other oil, ghee(Indian clarified butter-full flavor, stable at room temp and higher, 100% fat), dried coconut, maybe one 4 oz can of cooked crumbled bacon for a treat, whey protein powder, chocolate.
Compactness of food items is good though not 100% necessary for all things. Space is somewhat limited.
Crush the nuts and squash everything down very firmly when packing. Consolidate and squeeze air out reduces volume quite a bit, stick bars vertically along the perimeter of a bag. There's usually just enough space to slide them in.
Something to mask taste of water with higher salt/mineral content?
Crystal Lite, Koolaid(sweetened or unsweetened)
Canister stove or other?
Canister-well discussed by other posters.
Calories per day goals? Weight loss is assured but would like to keep it to 5kg max.
That depends a lot on your physiology, the terrain, total weight you have to propel, altitude. Do you have other trip data for a base line. Good place to start. Also, it might be worth while reading Ryan Jordan's discussion of the food for the Arctic1000 trip. Lots of good data. Salient points: They went in carrying a lot of extra body fat; Their meals for the first week weighed somewhere around 1.5 pounds as they burned body fat. By the third week, their food weighed ~3 pounds/day and contained ~7000 calories as they had burned all their body fat by then and had to depend on dietary calories. Worth pondering.
Another excellent brain to pick is Richard Nisley. You might try PM-ing him if he doesn't post to this thread. He has a wealth of info on the subject.