Covers: option-already-declared (duplicate mkOption), attribute-missing
(forgot to import nomarchy.nixosModules.system), Stylix target conflict
(needs lib.mkForce, not bare bool), home-manager .hm-bak churn (left over
from backupFileExtension after first install), and impermanence path
missing (dir not in environment.persistence list).
Each entry has the literal error text, the cause, and a copy-paste fix.
Linked from README.md and docs/MIGRATION.md so users hit it before
guessing.
