Add HVF→TCG fallback on macOS, force TCG in CI

GitHub-hosted macOS runners don't support HVF (QEMU aborts with
HV_UNSUPPORTED). DarwinBackend now auto-detects HVF via sysctl and
falls back to TCG, matching LinuxBackend's KVM detection pattern.
QEMU_ACCEL env var overrides detection for CI or debugging.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: pnc

.github/workflows/ci.yml

@@ -53,7 +53,7 @@ jobs:
 
   e2e-macos:
     runs-on: macos-latest
-    timeout-minutes: 15
+    timeout-minutes: 20
     steps:
       - uses: actions/checkout@v4
       - name: Install uv
@@ -68,6 +68,7 @@ jobs:
         run: uv run pytest tests/test_e2e.py -v -s
         env:
           VM_STATE_DIR: ${{ runner.temp }}/vm-state
+          QEMU_ACCEL: tcg  # HVF is unavailable on GitHub-hosted macOS runners
 
       - name: Upload logs on failure
         if: failure()

vm.py

@@ -147,18 +147,27 @@ def launch_qemu(self, qemu_args: list[str]) -> subprocess.Popen:
 # ---------------------------------------------------------------------------
 
 class DarwinBackend(Backend):
-    """macOS backend: HVF acceleration, Homebrew firmware paths."""
+    """macOS backend: HVF acceleration (with TCG fallback), Homebrew firmware paths."""
 
     def __init__(self, brew: Path, arch: Arch, proxy_port: int = PROXY_PORT,
                  ssh_host_port: int = SSH_HOST_PORT) -> None:
         super().__init__(arch, proxy_port, ssh_host_port)
         self._brew = brew
+        override = os.environ.get("QEMU_ACCEL")
+        if override:
+            self._accel = override
+        else:
+            r = subprocess.run(["sysctl", "-n", "kern.hv_support"],
+                               capture_output=True, text=True)
+            self._accel = "hvf" if r.returncode == 0 and r.stdout.strip() == "1" else "tcg"
 
     @property
     def machine_args(self) -> list[str]:
         if self.arch == Arch.ARM64:
-            return ["-machine", "virt,accel=hvf", "-cpu", "host"]
-        return ["-machine", "q35,accel=hvf", "-cpu", "host"]
+            cpu = "host" if self._accel == "hvf" else "cortex-a57"
+            return ["-machine", f"virt,accel={self._accel}", "-cpu", cpu]
+        cpu = "host" if self._accel == "hvf" else "qemu64"
+        return ["-machine", f"q35,accel={self._accel}", "-cpu", cpu]
 
     def prepare_efi(self, state_dir: Path) -> tuple[Path, Path]:
         code_src = self._brew / "share/qemu/edk2-aarch64-code.fd"