fix(blit): preserve source colors over transparency

[ishaanlabs-gg]

Summary

• use source-over alpha compositing when blitting pixels
• preserve source RGB values when compositing onto fully transparent destinations
• add a regression test for the gray-fringe case from Blitting uses wrong alpha compositing (generates borders) #1072

Fixes #1072.

Verification

• git diff --check
• node -e "const src={r:255,g:255,b:255,a:128}, dst={r:0,g:0,b:0,a:0}; const sa=src.a/255, da=dst.a/255, oa=sa+da*(1-sa); const c=(s,d)=>(s*sa+d*da*(1-sa))/oa; const actual=[c(src.r,dst.r),c(src.g,dst.g),c(src.b,dst.b),oa*255]; const expected=[255,255,255,128]; if (JSON.stringify(actual)!==JSON.stringify(expected)) { console.error({actual, expected}); process.exit(1); } console.log('alpha compositing regression math ok');"

I attempted pnpm --filter @jimp/plugin-blit test -- --watch=false, but the local dependency link could not complete on this machine because the external SSD ran out of free space (ERR_PNPM_ENOSPC).