Enlarge watermark badge for preview/encode parity and document video title fields.

Fix double-scaled badge preview, bump default watermark to 42% frame width, and update OSS docs/API for title vs songTitle separation and shared typography.

Co-authored-by: Cursor <cursoragent@cursor.com>

lib/preview-typography.ts

@@ -18,7 +18,7 @@ export function artistFontSizeForWidth(width: number): number {
 }
 
 export function watermarkFontSizeForWidth(width: number): number {
-  return Math.max(16, Math.round(width * 0.018));
+  return Math.max(22, Math.round(width * 0.028));
 }
 
 /** Scale encode-resolution px to preview container px. */
@@ -43,4 +43,4 @@ export function curatedFontApiUrl(key: string): string {
 }
 
 /** Watermark overlay width as fraction of frame (matches encode.ts). */
-export const WATERMARK_WIDTH_FRACTION = 0.32;
+export const WATERMARK_WIDTH_FRACTION = 0.42;