feat(image): auto-enable cross-arch chroot via qemu binfmt

Building a foreign-arch image (e.g. arm64 Raspberry Pi OS on an x86 host) needs
a qemu binfmt handler, or the chroot fails with "Exec format error". crossarch
detects this before the target is erased and, on confirmation, installs
qemu-user-static via the host package manager (apt-get/dnf/zypper/pacman) and
registers binfmt; otherwise it aborts with per-distro instructions.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

lim/image/crossarch.py

@@ -0,0 +1,71 @@
+"""Ensure the host can chroot into a foreign-architecture image (qemu binfmt)."""
+
+import platform
+import shutil
+from pathlib import Path
+
+from lim import runner, ui
+from lim.errors import LimError
+
+# Building natively needs no qemu; on these the chroot runs directly.
+_NATIVE_ARM = ("aarch64", "armv7l", "armv6l", "arm64")
+
+# Repo-based installs only; AUR helpers refuse to run from this root process.
+_BINFMT_INSTALLERS = {
+    "apt-get": ["apt-get", "install", "-y", "qemu-user-static", "binfmt-support"],
+    "dnf": ["dnf", "install", "-y", "qemu-user-static"],
+    "zypper": ["zypper", "--non-interactive", "install", "qemu-linux-user"],
+    "pacman": ["pacman", "-S", "--needed", "--noconfirm", "qemu-user-static-binfmt"],
+}
+
+
+def _entry_enabled(entry: Path) -> bool:
+    try:
+        return "enabled" in entry.read_text()
+    except OSError:
+        return False
+
+
+def qemu_binfmt_enabled() -> bool:
+    binfmt = Path("/proc/sys/fs/binfmt_misc")
+    return binfmt.is_dir() and any(_entry_enabled(e) for e in binfmt.glob("qemu-*"))
+
+
+def auto_enable() -> bool:
+    """Install qemu-user-static via the host package manager and register binfmt."""
+    command = next((cmd for tool, cmd in _BINFMT_INSTALLERS.items() if shutil.which(tool)), None)
+    if command is None:
+        ui.warning("No supported host package manager found (apt-get/dnf/zypper/pacman).")
+        return False
+    runner.run(command, sudo=True, check=False)
+    runner.run(["systemctl", "restart", "systemd-binfmt"], sudo=True, check=False)
+    return qemu_binfmt_enabled()
+
+
+def ensure_ready() -> None:
+    """Make foreign-arch chroot work, or fail with actionable guidance.
+
+    Called before the target is erased, so a host that cannot run the image's
+    binaries aborts early instead of mid-build with a cryptic "Exec format error".
+    """
+    if platform.machine() in _NATIVE_ARM:
+        return
+    if qemu_binfmt_enabled():
+        ui.info("Cross-architecture build: qemu binfmt handlers are registered.")
+        return
+    ui.warning(
+        f"Cross-architecture build: this host ({platform.machine()}) cannot run the "
+        "image's ARM binaries in chroot yet."
+    )
+    if ui.confirm("Install qemu-user-static and register binfmt now?"):
+        if auto_enable():
+            ui.success("qemu binfmt handlers registered.")
+            return
+        ui.warning("Automatic setup did not register a handler.")
+    raise LimError(
+        "Cross-architecture chroot is not available. Install qemu-user-static + binfmt "
+        "manually, or build on the target itself (e.g. the Pi booted from USB).\n"
+        "  Arch/Manjaro : qemu-user-static + qemu-user-static-binfmt (AUR), "
+        "then: sudo systemctl restart systemd-binfmt\n"
+        "  Debian/Ubuntu: sudo apt install qemu-user-static binfmt-support"
+    )