feat: add --kernel flag for QEMU system emulation mode

Add --kernel <PATH> option to boot extracted rootfs in a QEMU virtual
machine instead of namespace/chroot mode. The rootfs is converted to an
ext4 disk image using mke2fs and booted with the provided kernel.

README.md

@@ -38,6 +38,8 @@ ecr [OPTIONS] <DISTRO[:VERSION]> [-- COMMAND...]
 | `--no-cache` | Force a fresh download, bypassing the cache |
 | `-v, --verbose` | Print diagnostic output (URLs, layer info, extraction steps) |
 | `-a, --arch <ARCH>` | Target architecture (`amd64`, `arm64`, `armhf`, `riscv64`, …) |
+| `--kernel <PATH>` | Boot with QEMU system emulation using specified kernel |
+| `-m, --memory <SIZE>` | Memory for QEMU VM (default: 2G, only with `--kernel`) |
 
 ## Examples
 
@@ -56,8 +58,33 @@ ecr --arch arm64 alpine -- uname -m
 
 # Always pull a fresh image
 ecr --no-cache fedora
+
+# Boot with QEMU system emulation (requires qemu-system-<arch> and e2fsprogs)
+ecr --kernel /boot/vmlinuz ubuntu
+
+# Boot with custom memory
+ecr --kernel /boot/vmlinuz --memory 4G alpine
 ```
 
+## QEMU System Mode
+
+When `--kernel` is specified, ecr boots the rootfs in a full QEMU virtual machine instead of using namespaces:
+
+```sh
+ecr --kernel /boot/vmlinuz alpine
+```
+
+This mode:
+- Creates an ext4 disk image from the rootfs
+- Boots QEMU with your kernel
+- Provides full VM isolation
+- Works for any architecture (no binfmt_misc needed)
+
+Requirements:
+- `qemu-system-<arch>` installed
+- `e2fsprogs` for disk image creation
+- Kernel with virtio support
+
 ## Supported distributions
 
 | Name | Source | Version examples |