Combine 8 oz of the raspberries, 1/4 cup sugar, and lemon juice in a small saucepan over medium heat. Cook for 8-10 minutes, mashing berries occasionally, until mixture thickens to a loose jam consistency. Strain through a fine-mesh sieve, pressing firmly to extract all liquid; discard seeds. Cool completely. This should yield about 1/2 cup smooth raspberry puree.

Blend the remaining 4 oz fresh raspberries with 1/4 cup sugar until completely smooth, about 1 minute. Strain through the fine-mesh sieve into a large bowl, pressing to extract all juice. You should have about 1/3 cup vibrant pink liquid. Discard seeds.

Whisk the raspberry juice, heavy cream, whole milk, remaining 1/4 cup sugar, rosewater, and salt together until sugar dissolves, about 1 minute. Taste carefully - rosewater should be present but subtle; add drop by drop if needed. The mixture should taste slightly too sweet and too floral; freezing mutes both.

Cover and refrigerate the base for at least 4 hours or overnight until very cold, ideally 40F or below. A cold base churns faster with smaller ice crystals and better texture.

Churn the cold base in your ice cream maker according to manufacturer's instructions until it reaches soft-serve consistency, typically 20-25 minutes. The mixture will thicken and lighten in color to a pale pink.

Spread one-third of the churned ice cream into a 9x5 inch loaf pan or freezer container. Drizzle with one-third of the cooled raspberry puree. Repeat layers twice more. Use a butter knife to drag through the layers 2-3 times, creating a loose swirl pattern. Do not overmix or you'll lose the marbled effect.

Press plastic wrap directly onto the surface and freeze until firm, at least 4 hours or up to 2 weeks. Let sit at room temperature for 5 minutes before scooping. Serve with chopped pistachios and dried rose petals scattered on top if desired.