bug: verify-artifacts helper checks skipped

[krystophny]

Summary

make verify-artifacts prints [command not found] errors for helper functions like check_png_size and check_pdf_ok, so the intended PNG/PDF sanity checks never run.

Steps to Reproduce

1. From the repo root run make verify-artifacts.
2. Observe the log lines check_png_size: command not found, check_pdf_ok: command not found, check_pdftotext_has: command not found.
3. The target still reports "Artifact verification passed" despite the helper checks failing to execute.

Expected Result

• Helper functions stay in scope for the entire recipe.
• set -euo pipefail propagates, so any failed helper call aborts the target.
• PNG/PDF/text checks actually run and gate the build.

Actual Result

• Each helper invocation happens in a fresh shell without the function definitions, so Bash prints command not found and continues.
• The guard checks silently skip, weakening artifact verification.

Notes

• Within the verify-artifacts recipe the comment lines (e.g. # A couple PNG size checks ...) lack a leading tab. Make treats them as Makefile comments rather than part of the shell script, so the recipe breaks into multiple shell invocations.
• Replacing those # lines with tab-prefixed comments (or moving them inside a : <<'EOF' block) should keep the helpers and set -euo pipefail in scope.