feat: add core FSDK printer payload

[castrojo]

Summary

• build the missing pyppd and Foomatic XML tooling, then generate the three runtime PPD archives
• compose the FSDK-owned CUPS, Ghostscript, cups-filters, libppd, Python, xz, and core filter/backend payload without introducing a second CUPS provider
• fix the installed test-page name and avoid libcups' inherited option-string lock when external filters/backends run after fork()
• exercise a real Generic PCL 6/PCL XL Foomatic conversion through PAPPL and the CUPS socket backend

Verification

• just validate
• just verify-core
• just verify-payload
• just verify-cups-patch-chain
• actionlint .github/workflows/*.yml
• bash -n files/container-entrypoint.sh tests/core-appliance.sh tests/core-payload.sh tests/cups-patch-chain.sh
• git diff --check

The OCI image remains local and unpublished. This is a stacked PR based on feat/fsdk-core-app (#1).