Fix bosses_used compatibility on save load

[mleaf233]

Additional Info:

• I didn't modify api's
• I didn't modify api's
• I didn't make new lovely files

This restores compatibility for G.GAME.bosses_used after loading saves.

Custom Small/Big Blinds changed bosses_used from a flat table into a typed table:

  {
    boss = {},
    small = {},
    big = {},
  }

New runs get a metatable that preserves old flat-key access like:

G.GAME.bosses_used["bl_wall"]

However, save serialization does not preserve metatables. After loading a save, old mods that access
G.GAME.bosses_used["bl_*"] receive nil, even when the value exists under G.GAME.bosses_used.boss.

This patch normalizes loaded bosses_used tables and restores the compatibility metatable after Game:start_run loads
save data. keeps the new typed structure while preserving the old flat access pattern used by existing mods built against
vanilla Balatro / earlier SMODS behavior.