Reverts 4 commits (8139911c, 282f3aff, b3aca96d, dad61e4e) that
tried to workaround slow CI by limiting data subsets and cleaning
caches. The real issue is debug-mode .dbn.zst parsing taking minutes.
Kept: --test-threads=1 fix (root cause), hard CUDA error, action
range fixes, #[ignore] for heavy tests.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>