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>

tests/unit/test_cross_arch.py

@@ -0,0 +1,50 @@
+import pytest
+
+from lim.errors import LimError
+from lim.image import crossarch
+
+
+class TestCrossArch:
+    def test_native_arm_is_ok(self, monkeypatch):
+        monkeypatch.setattr(crossarch.platform, "machine", lambda: "aarch64")
+        crossarch.ensure_ready()
+
+    def test_cross_arch_with_binfmt_ok(self, monkeypatch):
+        monkeypatch.setattr(crossarch.platform, "machine", lambda: "x86_64")
+        monkeypatch.setattr(crossarch, "qemu_binfmt_enabled", lambda: True)
+        crossarch.ensure_ready()
+
+    def test_declined_auto_install_raises(self, monkeypatch, answers):
+        monkeypatch.setattr(crossarch.platform, "machine", lambda: "x86_64")
+        monkeypatch.setattr(crossarch, "qemu_binfmt_enabled", lambda: False)
+        answers("n")
+        with pytest.raises(LimError, match="Cross-architecture"):
+            crossarch.ensure_ready()
+
+    def test_auto_install_success_proceeds(self, monkeypatch, answers):
+        monkeypatch.setattr(crossarch.platform, "machine", lambda: "x86_64")
+        monkeypatch.setattr(crossarch, "qemu_binfmt_enabled", lambda: False)
+        monkeypatch.setattr(crossarch, "auto_enable", lambda: True)
+        answers("y")
+        crossarch.ensure_ready()
+
+    def test_auto_install_failure_raises(self, monkeypatch, answers):
+        monkeypatch.setattr(crossarch.platform, "machine", lambda: "x86_64")
+        monkeypatch.setattr(crossarch, "qemu_binfmt_enabled", lambda: False)
+        monkeypatch.setattr(crossarch, "auto_enable", lambda: False)
+        answers("y")
+        with pytest.raises(LimError, match="Cross-architecture"):
+            crossarch.ensure_ready()
+
+    def test_auto_enable_uses_detected_manager_and_registers(self, monkeypatch, fake_runner):
+        monkeypatch.setattr(
+            crossarch.shutil, "which", lambda tool: "/bin/pacman" if tool == "pacman" else None
+        )
+        monkeypatch.setattr(crossarch, "qemu_binfmt_enabled", lambda: True)
+        assert crossarch.auto_enable() is True
+        assert fake_runner.find("pacman", "-S", "qemu-user-static-binfmt")
+        assert fake_runner.find("systemctl", "restart", "systemd-binfmt")
+
+    def test_auto_enable_without_manager_returns_false(self, monkeypatch):
+        monkeypatch.setattr(crossarch.shutil, "which", lambda _tool: None)
+        assert crossarch.auto_enable() is False