Stir together 1 gallon cold water and cornmeal in a large bowl. Stir in clams; let stand, uncovered, at room temperature 1 hour. Drain; rinse clams under cold water, and drain again. Set clams aside.

While clams stand, heat 2 tablespoons oil in a medium-size nonstick skillet over medium-high. Reduce heat to medium; add panko, and cook, stirring often, until golden brown and toasted, 3 to 5 minutes. Stir in lemon zest and salt; cook, stirring constantly, until fragrant, about 30 seconds. Remove from heat. Set aside.

Bring a large pot of salted water to a boil over medium-high. Add spaghetti, and cook according to package directions for al dente. Drain pasta, reserving 1 cup pasta cooking liquid in a small heatproof bowl. Transfer pasta to a large rimmed baking sheet; set aside until ready to use

Heat remaining 2 tablespoons oil in a large, deep skillet or Dutch oven over medium-high. Add sausage, garlic, and gochugaru, if using. Cook, stirring often, until sausage is browned, 3 to 4 minutes. Stir in clams and wine. Reduce heat to medium, and cover. Cook, covered, until clams open, 5 to 6 minutes. Remove and discard any clams that have not opened. Add pasta and ½ cup reserved pasta cooking liquid to skillet. Cook, stirring vigorously, until sauce is emulsified and slightly thickened, about 2 minutes, adding more cooking water ¼ cup at a time if needed. Stir in parsley, and season with additional salt to taste. Sprinkle with toasted panko mixture, and top with lemon wedges. Serve immediately.