refactor: split main.rs into registry, build, and qemu modules

main.rs keeps only the CLI definition and command dispatch:
- registry.rs: image-name validation + XDG registry path handling (tests)
- build.rs: podman mount/unmount, vmlinuz extraction, cpio/gzip initrd
- qemu.rs: direct-kernel-boot VM launch

No behavior change; cargo fmt/clippy/test pass, build/run paths
verified against the mock-vm image.

src/qemu.rs

@@ -0,0 +1,56 @@
+//! QEMU: boot a registry entry with qemu-system-x86_64 using direct kernel
+//! boot (-kernel/-initrd).
+
+use anyhow::{Context, Result, bail};
+use std::process::Command;
+
+use crate::registry::registry_dir;
+
+pub(crate) fn run(name: &str) -> Result<()> {
+    let reg_dir = registry_dir(name)?;
+    let vmlinuz_path = reg_dir.join("vmlinuz");
+    let initrd_path = reg_dir.join("initrd");
+    if !vmlinuz_path.exists() || !initrd_path.exists() {
+        bail!(
+            "Registry missing vmlinuz/initrd for '{}'. Run `slim build` first.",
+            name
+        );
+    }
+    println!("Booting {} from registry: {}", name, reg_dir.display());
+    println!(
+        "  qemu-system-x86_64 -m 256 -nographic -kernel {} -initrd {} -append 'console=ttyS0'",
+        vmlinuz_path.display(),
+        initrd_path.display()
+    );
+
+    let cmdline = [
+        "console=ttyS0,115200",
+        "root=/dev/ram0",
+        "rw",
+        "earlyprintk=serial",
+        "nokaslr",
+        "raid=noautodetect",
+    ]
+    .join(" ");
+
+    let status = Command::new("qemu-system-x86_64")
+        .args(["-m", "1024M"])
+        .arg("-kernel")
+        .arg(vmlinuz_path.as_os_str())
+        .arg("-initrd")
+        .arg(initrd_path.as_os_str())
+        .args(["-append", &cmdline])
+        .args(["-nographic"])
+        .status()
+        .context("Failed to launch qemu-system-x86_64")?;
+
+    match status.code() {
+        Some(0) => println!("QEMU exited cleanly."),
+        Some(126) | Some(127) => {
+            bail!("Failed to start qemu-system-x86_64 (is it installed and in PATH?)")
+        }
+        Some(code) => bail!("QEMU exited with code {code}"),
+        None => bail!("QEMU terminated by a signal"),
+    }
+    Ok(())
+}