Heat the oven to 325°F (165°C). Line a 9x9-inch baking pan with parchment paper, leaving overhang on two sides for easy lifting.
Make the cranberry swirl: combine the cranberries, sugar, orange juice, and orange zest in a small saucepan over medium heat. Cook, stirring often, until the cranberries burst and the mixture thickens, about 8 to 10 minutes. Stir in the cornstarch slurry and cook 1 minute more. Let it cool for 10 minutes, then blend briefly or mash with a fork for a spoonable sauce with a few whole berries left.
Make the crust: stir the graham cracker crumbs, sugar, melted butter, and salt in a medium bowl until the crumbs look like wet sand. Press firmly and evenly into the bottom of the prepared pan. Bake for 8 minutes, then set aside to cool slightly.
Make the filling: beat the cream cheese and sugar in a large bowl on medium speed until completely smooth and fluffy, about 2 to 3 minutes, scraping the bowl halfway through. Beat in the eggs one at a time, then the sour cream, vanilla, flour, and salt, mixing just until combined. Do not overbeat — excess air causes cracks.
Pour the filling over the warm crust and smooth the top with an offset spatula.
Spoon the cooled cranberry sauce over the filling in dollops, then drag a toothpick or knife through the dollops in long S-curves to create a marbled swirl. Do not over-swirl or the colors will muddy.
Bake for 30 to 35 minutes, until the edges are set and the center still has a slight jiggle when you gently shake the pan. Turn off the oven, crack the door, and let the bars cool inside for 30 minutes — this gentle cool-down prevents cracks.
Remove from the oven and cool completely at room temperature, about 1 hour. Cover and refrigerate for at least 4 hours, or overnight, until fully firm.
Lift the bars out of the pan using the parchment overhang. Slice into 16 squares with a long, sharp knife, wiping the blade clean between cuts for neat edges.
Top with sugared cranberries and a drizzle of melted white chocolate if you like. Serve chilled or at cool room temperature.