Add --mount flag for 9p host directory sharing

Add support for sharing host directories into the VM via QEMU 9p
(virtio-9p).

The --mount flag can be repeated.

The format is `<host-path>` or `<host-path>:<guest-path>`

Relative guest paths are relative to the configured WORKDIR.

Design:
- QEMU: each --mount gets a short 9p tag (slim0, slim1, …) via
  -virtfs local,path=…,mount_tag=slimN,security_model=mapped-xattr.
  Tags are kept short because 9p mount_tag has a ~31-byte limit.
- Kernel cmdline: the full destination path is passed as
  slim.mount=<tag>:<base64(path)> so the init script knows where to
  mount each tag.  Base64 avoids issues with spaces/special chars.
- slim-init.sh: after networking, parse slim.mount= entries, mkdir -p
  the destination, and mount -t 9p <tag> <dest> -o trans=virtio,version=9p2000.L

Tests verify: 9p share detection via sysfs mount_tag, 9p entry in
mount output, file content accessible at the expected path, and clean
VM exit.

src/build.rs

@@ -78,7 +78,12 @@ fn build_inner(
     let command = cmd.unwrap_or_else(|| inject::infer_command(&config));
     let exec_script =
         inject::build_exec_script(&command, &config.env, config.working_dir.as_deref());
-    inject::inject(mount_path, &exec_script, config.user.as_deref())?;
+    inject::inject(
+        mount_path,
+        &exec_script,
+        config.user.as_deref(),
+        config.working_dir.as_deref(),
+    )?;
     println!("Injected /slim/ (init + exec)");
 
     match kind {