The PCI scan from ring 3: pci-bus walks the ECAM it mapped (M19.1)

The manager matches the pci_host_bridge node and spawns pci-bus with the
bridge id as its assignment — hello, supervision, restart, all the M18
contract for free. The driver claims the bridge, maps the ECAM window
(resource 0) through the ordinary mmio grant, and repeats the kernel's
brute-force bus/device/function walk from user space. The pci-scan
scenario builds its expected marker from the kernel's own function count,
so the two enumerations must agree exactly — the equivalence that
licenses retiring the kernel walk in M19.3.

system/services/device-manager/device-manager.zig

@@ -318,6 +318,13 @@ fn initialise(endpoint: runtime.ipc.Handle) bool {
 
     var matched: usize = 0;
     for (buffer[0..count]) |descriptor| {
+        if (descriptor.class == @intFromEnum(device.DeviceClass.pci_host_bridge)) {
+            // The PCI bus driver: enumeration in ring 3 (M19), one instance
+            // per bridge, the bridge id as its assignment.
+            matched += 1;
+            addDriver("pci-bus", descriptor.id, true);
+            continue;
+        }
         if (pciDriverFor(descriptor)) |driver_name| {
             matched += 1;
             addDriver(driver_name, descriptor.id, true);