fix(images): reject non-printable core Flatpak names

[bsherman]

Summary

Keeps snosi's producer validation in step with frostyard/firn#111. Firn's wizard renders each core app's name on the installer console from an unverified preview inspection, so firn now treats names containing control or format characters (escape sequences, newlines, tabs, zero-width joiners) as malformed. check_set now rejects the same names, so snosi can't publish a label that every opted-in firn install refuses.

str.isprintable() matches Go's unicode.IsPrint, which firn uses: plain spaces and non-ASCII letters pass. Every published label today is plain ASCII, and validate passes unchanged.

Type of change

• Build pipeline / CI change

Validation

• shellcheck / validate.yml static checks pass for touched scripts
• Relevant tests in test/ were run

Commands run:

./test/core-flatpaks-test.sh        # 60 passed, 0 failed (new: escape, newline, tab, ZWJ rejected; accented name accepted)
./flatpaks/core-flatpaks.py validate
shellcheck -S warning -x test/core-flatpaks-test.sh

Checklist

• Changes are as small and focused as possible
• No secrets, keys, or credentials are committed

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