Use a `FROM scratch AS debuginfo` stage instead of copying the .debug
file to the final image. This keeps the shipped image at exactly the
same size as before. CI extracts the .debug file using depot's
--target debuginfo with cache hits from the main build.
Also adds gnu_debuglink so gdb auto-discovers the debug file when
placed next to the binary.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>