Fix build changed count always showing total packages

[retlehs]

Summary

• The "Changed" column on the admin builds page has been showing the
  total package count (61403) since the Composer v1 removal/rename
• The old builder used content-addressed p/ filenames for change
  detection; when p/ was removed, the detection was lost
• Restores PreviousBuildDir on BuildOpts and compares each p2/
  file against the previous build via byte comparison
• Display-only fix — R2 sync already has its own change detection and
  is unaffected

Test plan

• make test passes
• Deploy and verify next build shows a realistic changed count

🤖 Generated with Claude Code