补busybox

docs: sync EXT4 status for busybox P0 syscall and overlay work

Update EXT4_STATUS with the latest busybox P0 filesystem/syscall work. Document the referenced rCore and Linux kernel source files, including getcwd/chdir, openat, getdents64, mkdirat, unlinkat, read/write/lseek, execve/wait4, and fd/task structure references.

Record the current EXT4 read-only plus overlay writable model, cwd inheritance behavior, runner command logging, and latest QEMU validation results for pwd/ls/cat/mkdir/cp/rm/sh/dd smoke paths.