Draft v5.0.3

Fixed

• fix-whitespace.sh truncated Markdown files to one byte on macOS. Its
  trailing-blank-line collapse used a GNU-only sed construct; BSD sed
  rejected it, the empty output was written back over the file, and the run
  still reported success. Decompose's cleanup step and condensation
  (architecture.md, .ai-context.md, .ai-profile.md) both call it. The
  collapse is now portable awk, and the tool refuses to replace a file that
  has content with whitespace-only output.
• check-track-hygiene.sh crashed on any track with a plan.md but no
  hld.md or lld.md — unbound variable, on every platform. The
  plan-staleness check now treats a missing sibling as nothing to compare.