fix(export): scale mp4 bitrate with frame rate

src/lib/exporter/exportBitrate.test.ts

@@ -15,6 +15,28 @@ describe("export bitrate policy", () => {
 		).toBe(27_000_000);
 	});
 
+	it("raises high-resolution 60fps source-quality exports above the 30fps budget", () => {
+		const sharedOptions = {
+			width: 2560,
+			height: 1440,
+			quality: "source" as const,
+			encodingMode: "quality" as const,
+		};
+
+		const thirtyFpsBitrate = getMp4ExportBitrate({
+			...sharedOptions,
+			frameRate: 30,
+		});
+		const sixtyFpsBitrate = getMp4ExportBitrate({
+			...sharedOptions,
+			frameRate: 60,
+		});
+
+		expect(thirtyFpsBitrate).toBe(45_000_000);
+		expect(sixtyFpsBitrate).toBeGreaterThan(thirtyFpsBitrate);
+		expect(sixtyFpsBitrate).toBe(63_639_610);
+	});
+
 	it("keeps modern native static-layout source exports high enough for screen text", () => {
 		expect(
 			getMp4ExportBitrate({

src/lib/exporter/exportBitrate.ts

@@ -2,6 +2,7 @@ import type { ExportEncodingMode, ExportMp4FrameRate, ExportQuality } from "./ty
 
 const MIN_MP4_BITRATE = 2_000_000;
 const REFERENCE_PIXEL_RATE = 1920 * 1080 * 30;
+const REFERENCE_FRAME_RATE = 30;
 
 export function getEncodingModeBitrateMultiplier(encodingMode: ExportEncodingMode): number {
 	switch (encodingMode) {
@@ -41,6 +42,10 @@ function getBaseMp4ExportBitrate(width: number, height: number, quality: ExportQ
 	return 30_000_000;
 }
 
+function getFrameRateBitrateMultiplier(frameRate: ExportMp4FrameRate): number {
+	return Math.sqrt(Math.max(1, frameRate / REFERENCE_FRAME_RATE));
+}
+
 function getModernNativeStaticLayoutBitrateCap(
 	width: number,
 	height: number,
@@ -87,6 +92,7 @@ export function getMp4ExportBitrate(options: {
 }): number {
 	const requestedBitrate = Math.round(
 		getBaseMp4ExportBitrate(options.width, options.height, options.quality) *
+			getFrameRateBitrateMultiplier(options.frameRate) *
 			getEncodingModeBitrateMultiplier(options.encodingMode),
 	);
 	const nativeStaticLayoutBitrate =