Add deep extent tree support (depth >= 2) in EXT4 formatter

[gbfish]

Summary

• Reader: Refactor getExtents() with recursive readExtentNode() that handles any depth
• Formatter: Extract writeExtentSubtree() that recursively builds extent trees of any depth
• Fix force-unwrap crash on leafNode.leaves.last!.block (use safe optional access)
• Replace hardcoded depth 0/1 switch with general depth calculation
• Consistent use of .loadLittleEndian(as:) for all on-disk struct reads
• Add test for multi-extent file readback

[dcantah]

Thanks for the work. To me, would you be open to merging the two changes into one? It'd be simpler to look for logical bugs if the formatter and reader are all in one.

[gbfish]

Done — combined both reader and formatter changes into this single PR. Thanks for the suggestion.