feat: ext2 filesystem driver with full RW support

feat: ext2 filesystem driver with full RW support

- New ext2.c + ext2.h: complete ext2 filesystem implementation
- Superblock parsing, block group descriptor table, inode read/write
- Block mapping: direct, singly/doubly/triply indirect blocks
- File read/write with automatic block allocation
- Directory operations: finddir, readdir, create, mkdir, unlink, rmdir,
  rename, truncate, link (hard links)
- Block and inode bitmap allocation/deallocation
- Symlink support (inline small symlinks via i_block)
- Auto-detection of inode size (128 or 256 for rev1)
- Supports 1KB, 2KB, and 4KB block sizes

Build: clean, cppcheck: clean, smoke: 19/19 pass