fix(e2fs): decode libext2fs errors with Ext2Code

[JingMox]

Description

Fix incorrect libext2fs error-code descriptions by replacing the raw offset-based table with a type-safe Ext2Code enum.

Changes

• Add Ext2Code to classify libext2fs errors and bare host errno values.
• Correct error descriptions against ext2_err.h from e2fsprogs 1.47.1.
• Decode errors once in check() instead of exposing raw i64 codes.
• Update mkdir_p to match Ext2Code::DirExists.
• Export Ext2Code from bux-e2fs.
• Add unit tests for decoding, round-tripping, and error display.

Closes #56