Discovery-migration prerequisites (M19.0)

The host bridge now carries MMIO apertures derived from the boot memory
map's gaps below 4 GiB (largest three, sort-merged; a single after-the-
last-region hole dies on OVMF's flash at the top) plus one aperture above
the described space — so a user-space device_register of PCI functions
with BAR resources can pass containment. The discovery test asserts
every PCI memory resource lies inside a bridge window and names any
escapee. device_register is idempotent on exact (parent, class, identity,
resources) match — a restarted registering bus cannot duplicate its
children; proven directly against the broker in the bus test.
ChildAdded gains device_id so a report can carry the registered kernel
id a matched driver needs as its assignment.

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

@@ -73,8 +73,13 @@ pub const ChildAdded = extern struct {
     parent: u64,
     /// Where on the bus (for USB: the root port number, 1-based).
     bus_address: u64,
-    /// Bus-specific identity (for USB: the PORTSC port-speed class).
+    /// Bus-specific identity (for USB: the PORTSC port-speed class; for PCI:
+    /// the class triple).
     identity: u64,
+    /// The kernel device id this child was `device_register`ed as — what the
+    /// manager hands a matched driver as its argv assignment — or `no_device`
+    /// for an unregistered leaf (a USB port before the descriptor track).
+    device_id: u64 = no_device,
 };
 
 pub const child_added_size = @sizeOf(ChildAdded);