PreparationChicken: Combine garlic, ginger, citrus juice, soy sauce, vinegar, and sugar in a medium bowl. Place chicken halves in a large resealable plastic bag and pour in marinade. Seal bag, pressing out any air. Chill at least 12 hours and up to 2 days. Place rack in top third of oven; preheat to 425°F. Remove chicken from bag; discard marinade. Pat chicken dry with paper towels; season lightly with salt. Roast chicken on a foil-lined rimmed baking sheet until deep brown and cooked through, 30–40 minutes. Dipping sauce and assembly: Bruise lemongrass stalk by lightly crushing with a rolling pin. Bring lemongrass and 1/2 cup water to a boil in a small saucepan. Remove from heat and let sit until very fragrant, 8–10 minutes; strain lemongrass infusion into a large measuring glass. Meanwhile, heat a large skillet, preferably cast iron, over medium-high. Toast shallots, scallions, chile, and garlic, tossing occasionally, until dark brown in spots, 8–10 minutes. Let cool. Remove skins from garlic and coarsely chop garlic with shallots, scallions, and chile. Add garlic, shallots, scallions, chile, and ginger to lemongrass infusion. Stir in fish sauce, lime juice, and sugar. Using a sharp knife, remove peel and white pith from pomelo; discard. Working over a medium bowl, cut along sides of membranes to release segments. Squeeze juice from membranes into bowl; discard. Tear segments into large pieces. Mix into dipping sauce. Transfer chicken to a platter, spoon some dipping sauce over, and top with cilantro. Serve with remaining dipping sauce alongside.