The 6.19 font cleanup removes Sun-ExtB from sites that use Sun-ExtA #2025

Problem

The 6.19 core font cleanup deletes Sun-ExtB.ttf from a site that uses Sun-ExtA, and nothing downloads it again. On a 6.18 site with the old installer's fonts, Sun-ExtA supplies CJK text and Sun-ExtB supplies the CJK Extension B characters (for example 𠀀), so those characters are likely to stop rendering after the upgrade.

Steps to reproduce

1. Put the mpdf-core-fonts v1.0.0 Sun-ExtA.ttf and Sun-ExtB.ttf in wp-content/uploads/PDF_EXTENDED_TEMPLATES/fonts/
2. Set the default font to Sun-ExtA
3. Set the gfpdf_current_version option to 6.18.0 and load any admin page
4. Run wp cron event run gfpdf_core_font_cleanup_run, then wp cron event run gfpdf_core_font_sync

Sun-ExtA.ttf is back at the current build, and Sun-ExtB.ttf is gone. Reproduced on the #2011 branch. The missing characters were read from the code rather than rendered.

Cause

Model_Required_Fonts::get_font_files() reads only a family's R, B, I and BI files, so sun-exta's sip-ext link to sun-extb isn't counted. Sun-ExtB is never in the required list, so prune() deletes the unmodified v1.0.0 copy and sync() doesn't fetch it. Model_Font_Sources then removes the sip-ext link and doesn't use Sun-ExtB as backupSIPFont, as the file isn't on disk.

This comes from #1998's "keep only what's needed" rule. #2011 doesn't change it: Sun-ExtB is deleted because it isn't needed, not because it's outdated.

Relationship to #2014

With #2014 merged, a CJK Extension B character queues Sun-ExtB for the background sync, so the site recovers. That PR's first render of such a character still comes out degraded, and until it merges the file doesn't come back at all.

Proposal

Count a family's sip-ext font as part of the family in get_font_files(), so a site that needs Sun-ExtA keeps Sun-ExtB. With #2011 that file is then replaced with the repaired build like Sun-ExtA.

The cost is another 17.5 MB download for sites that need Sun-ExtA, including ones that never use Extension B characters. The alternative is to keep only an existing v1.0.0 Sun-ExtB through the cleanup when Sun-ExtA is needed, and leave new installs to #2014's on-demand path.