V321 - NATIVE PLAYER-CHOSEN EXPORT NAME + FIVE-LETTER ACCOUNT IMPORT

Built from V320. V319 stock Import/Export screens and V317/V318 multiplayer,
account, death/menu, story, and pause logic are retained.

Issue addressed:
  BG2 allows a player to type an export filename (e.g. MyHero.CHR).
  V320 incorrectly required the physical source file itself to already start
  with the five-letter account prefix. This hid existing legitimate characters.

V321 behavior:
  - Keep BG2's native Export filename prompt unchanged.
  - Existing real CHR files in characters/<Account>/, including old source names,
    are eligible if they pass the active account's ownership checks.
  - No arbitrary source filename appears directly in native Import. The router
    presents only verified five-letter numbered aliases, such as drakh01.CHR
    for Drakhysys, krona01.CHR for Kronas, and drakk01.CHR for Drakkar.
  - If the original name is Drakhy.CHR, dra01.CHR, or MyHero.CHR, these names
    are NEVER imported directly: only the account-owned alias is shown.
  - New native exports with player-chosen filenames still use five-letter
    numbered canonical account files on adoption.
  - Explicitly foreign names (e.g. Drakhy.CHR under Kronas) are rejected;
    other known foreign numbered aliases and the stock pregenerated names
    Lessa/Rothgar/Abdel/Daria are not mounted.
  - Original CHR/BIO files are never modified, renamed, deleted, or moved during
    mounting. Only authenticated temporary native views are removed at logout.
  - V320's prefix and numbering registries are reused; V319 safe ZIP is untouched.

Install with BG2 closed using SETUP_IN_GAME_ACCOUNTS.bat, then InfinityLoader.exe.
Verify inside BG2; automated tests cannot fully simulate the engine's native
character preview list or the real multiplayer router process on Windows.
