feat: use cpio crate for initramfs creation

Add the `cpio` crate as dependency, removing e2fsprogs external dependency.

SPEC.md

@@ -187,7 +187,7 @@ No action required. Modern qemu-user-static packages register binfmt_misc with t
 
 ## QEMU System Emulation Mode
 
-When `--kernel` is specified, ecr switches from namespace/chroot mode to QEMU system emulation. The extracted rootfs is converted to a disk image and booted with the provided kernel.
+When `--kernel` is specified, ecr switches from namespace/chroot mode to QEMU system emulation. The extracted rootfs is converted to a gzipped CPIO initramfs and booted with the provided kernel.
 
 ### Usage
 
@@ -201,24 +201,20 @@ ecr --kernel /boot/vmlinuz debian -- /bin/sh -c "echo hello"
 
 1. Download/cache rootfs tarball (same as namespace mode)
 2. Extract tarball to temporary directory
-3. Create ext4 disk image from rootfs using `mke2fs -d` (requires `e2fsprogs`)
+3. Create gzipped CPIO initramfs from rootfs
 4. Launch QEMU with:
    - `-kernel <path>` - provided kernel
-   - `-append "root=/dev/vda rw console=ttyS0"` - kernel command line
+   - `-initrd initramfs.cpio.gz` - rootfs as initramfs
+   - `-append "console=ttyS0 rdinit=/bin/sh"` - kernel command line
    - `-m <memory>` - memory size (default 2G)
-   - `-nographic` - console on stdio
-   - `-drive file=rootfs.img,format=raw,if=virtio` - rootfs disk
+   - `-display none -serial mon:stdio` - console on stdio
    - `-netdev user,id=net0 -device virtio-net-pci,netdev=net0` - network
 5. Wait for QEMU to exit
 6. Cleanup temporary files
 
-### Disk Image Creation
+### Initramfs Creation
 
-The rootfs directory is converted to an ext4 disk image using `mke2fs -t ext4 -d <rootfs>`. This requires the `e2fsprogs` package:
-
-- Ubuntu/Debian: `sudo apt install e2fsprogs`
-- Arch: `sudo pacman -S e2fsprogs`
-- Alpine: `sudo apk add e2fsprogs`
+The rootfs directory is converted to a gzipped CPIO archive (newc format) using  the `cpio` crate.
 
 ### Architecture Support
 
@@ -234,8 +230,7 @@ The rootfs directory is converted to an ext4 disk image using `mke2fs -t ext4 -d
 ### Requirements
 
 - QEMU system emulator installed (`qemu-system-<arch>`)
-- `e2fsprogs` for disk image creation
-- Kernel with virtio support (for disk and network drivers)
+- Kernel with required drivers (serial console, virtio-net for network)
 
 ### Differences from Namespace Mode