feat(firecracker): persistent registry volume for the infra VM (Stage B, 6/n)

The infra VM's rootfs is ephemeral (rebuilt each boot), so the bottle
registry DB needs durable storage across restarts. Give the infra VM a
firecracker analogue of a docker volume: a host-side ext4 file attached as
a second virtio-block device (guest /dev/vdb), mounted at the control
plane's DB dir (/var/lib/bot-bottle, where host_db_path lives at
db/bot-bottle.db).

- firecracker_vm.boot/_config take an optional `data_drive` (a non-root,
  RW second drive).
- infra_vm creates the volume on first use (`mke2fs` an empty ext4 at
  <fc-cache>/infra/registry.ext4) and mounts /dev/vdb in the PID-1 init
  before the control plane starts. It's a plain ext4 file, so
  `sudo mount -o loop <path>` (VM stopped) inspects bot-bottle.db directly.

Verified on a KVM host: register a bottle, restart the infra VM (fresh
rootfs, same volume) — /dev/vdb re-mounts and the registry `db/` dir + a
marker file survive the restart.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01WBMWTEtQdJ4W5UrWuLHCck

bot_bottle/backend/firecracker/infra_vm.py

@@ -165,6 +165,7 @@ def boot() -> InfraVm:
         name="bot-bottle-infra", rootfs=rootfs, tap=slot.iface,
         guest_ip=slot.guest_ip, host_ip=slot.host_ip, pubkey=pubkey,
         run_dir=run_dir, mem_mib=4096, detached=True,
+        data_drive=_ensure_registry_volume(),
     )
     _pid_file().write_text(str(vm.process.pid))
     return InfraVm(guest_ip=slot.guest_ip, private_key=private_key, vm=vm)
@@ -180,6 +181,35 @@ def _pid_file() -> Path:
     return _infra_dir() / "vm.pid"
 
 
+# The registry "volume": a host-side ext4 file attached to the infra VM as a
+# second virtio-block device (guest /dev/vdb), mounted at the control plane's
+# DB dir. It outlives the ephemeral rootfs, so the bottle registry survives an
+# infra-VM restart — the firecracker analogue of a docker volume. It is a
+# plain ext4 file: `sudo mount -o loop <path>` on the host (with the VM
+# stopped) to inspect bot-bottle.db directly.
+_REGISTRY_SIZE = "512M"
+
+
+def registry_volume_path() -> Path:
+    return _infra_dir() / "registry.ext4"
+
+
+def _ensure_registry_volume() -> Path:
+    """Create the empty ext4 registry volume on first use; reuse it after."""
+    vol = registry_volume_path()
+    if vol.exists():
+        return vol
+    info(f"creating infra registry volume {vol} ({_REGISTRY_SIZE})")
+    proc = subprocess.run(
+        ["mke2fs", "-q", "-t", "ext4", "-F", str(vol), _REGISTRY_SIZE],
+        capture_output=True, text=True, check=False,
+    )
+    if proc.returncode != 0:
+        vol.unlink(missing_ok=True)
+        die(f"creating registry volume failed: {proc.stderr.strip()}")
+    return vol
+
+
 def _stable_keypair() -> tuple[Path, str]:
     """The infra VM's SSH keypair — generated once and reused, so any later
     launcher can SSH in (fetch CA / provision) even though a different process
@@ -321,6 +351,11 @@ if [ -n "$KEY" ]; then
 fi
 chown -R 0:0 /root 2>/dev/null || true
 mkdir -p /etc/dropbear /run /var/lib/bot-bottle
+
+# Persistent registry volume (second virtio-block device, /dev/vdb) mounted
+# at the control plane's DB dir, so bot-bottle.db survives infra-VM restarts.
+mount -t ext4 /dev/vdb /var/lib/bot-bottle 2>/dev/null || true
+
 /bb-dropbear -R -E -p 22 &
 
 # Control plane. Source is baked at /app; the package is stdlib-only.