GottZ triage (PR #76463) correctly identified that the sequential
clear+refresh per terminal could let a later clear invalidate an earlier
terminal's rebuilt glyph model. Split into two phases: all atlas clears
run first, then all terminal refreshes, guaranteeing every terminal
rebuilds against a clean shared atlas.
Added behavior-oriented regression test proving clear-before-refresh
ordering across multiple terminals sharing one atlas.
1dc3edf627