runtime: consolidate the device-manager hello into runtime.device_manager

Every supervised driver owes the device manager a hello at startup, and
the retry-lookup-call-check for it had been copied into five drivers:
usb.helloManager (misfiled in the USB client) plus hand-rolled twins in
display, virtio-gpu, pci-bus, and usb-xhci-bus.

Extract it once as a runtime client, runtime.device_manager.hello(role,
device_id) ?Handle — returns the manager endpoint (bus drivers keep it to
report children through), null when there is no manager or it refused the
handshake, and logs the outcome itself so each call site is one line.

Also correct two roles while collapsing their calls: virtio-gpu and the
display driver each claim one PCI function and report no children, so they
are Role.device, not Role.bus. The manager ignores role today, so this is
cosmetic, but it matches the protocol's own definition (bus = reports
children via child_added).

Behavior-preserving otherwise: pci-bus and usb-xhci-bus move their hello
logging from raw serial writes to std.log, which the kernel renders with
the same "<path>: " prefix, so driver-restart still matches
"usb-xhci-bus: hello acknowledged". zig build clean; 8 QEMU cases pass
(driver-restart, pci-scan, usb-hid, usb-storage, virtio-gpu,
display-reattach, device-list, device-manager).

library/runtime/runtime.zig

@@ -23,6 +23,9 @@ pub const vfs_protocol = @import("vfs-protocol");
 
 /// The device-manager protocol: hello + tree reports (docs/device-manager.md).
 pub const device_manager_protocol = @import("device-manager-protocol");
+/// Client for talking to the device manager (the hello handshake a supervised
+/// driver owes at startup). See library/runtime/device-manager.zig.
+pub const device_manager = @import("device-manager.zig");
 
 /// The power protocol: events (button, lid, battery) + shutdown (docs/power.md).
 pub const power_protocol = @import("power-protocol");