fix(cover): flatten transparencies by targeting PDF 1.3 compatibility

PDF 1.4 preserves transparency objects, which KDP's preflight rejects.
Downgrading to 1.3 forces Ghostscript to composite all rgba overlays,
gradient alphas and image opacities into solid pixels during the write.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

scripts/cover.js

@@ -50,7 +50,7 @@ async function runGhostscript({ rawPath, outputPath, marksPath, title, author, t
   const args = [
     '-dBATCH', '-dNOPAUSE', '-dQUIET',
     '-sDEVICE=pdfwrite',
-    '-dCompatibilityLevel=1.4',
+    '-dCompatibilityLevel=1.3',
     '-dPDFSETTINGS=/prepress',
     '-dEmbedAllFonts=true',
     '-dSubsetFonts=false',