V268 - ACCOUNT CREATED = characters_<accountname>

Permanent rule:
- CREATE ACCOUNT creates characters_<accountname>.
- That exact directory is the account folder from creation onward.
- Account-name spelling is preserved in the physical folder (Drakhysys -> characters_Drakhysys).
- Account identity remains case-insensitive, so Drakhysys/drakhysys cannot create separate accounts.
- LOGIN/Import/Export resolve the same permanent characters_<accountname> folder; no move, mirror copy, or replacement account folder is created.
- plain characters remains denied except for the temporary engine junction to the authenticated account folder.
- characters_<other account> remains denied.
- No automatic character backups or restoration are added.
- If folder creation is not confirmed, account creation is rolled back in Baldur.lua instead of leaving an account record without its folder.
