Decode the xHCI port speed in the usb-xhci-bus log

The root-hub scan logged the raw PORTSC port-speed class ("speed class 3").
Decode it to a human name — Low/Full/High/SuperSpeed/SuperSpeedPlus with the
USB generation and line rate — so the boot log says what enumerated on each
port, the USB analog of the pci-bus class line. This is the link speed only;
the device class/subclass/protocol needs descriptor reads (the USB track).

system/drivers/usb-xhci-bus/usb-xhci-bus.zig

@@ -108,6 +108,22 @@ fn readRegister(offset: usize) u32 {
     return register.*;
 }
 
+/// The xHCI default Protocol Speed IDs (the PORTSC port-speed field, bits 13:10)
+/// decoded to human names — the boot-log breadcrumb for what actually enumerated on
+/// a port, the USB analog of the pci-bus class-code line. A controller may redefine
+/// these through its Supported Protocol capability, but the defaults cover every
+/// speed QEMU and real hardware report at this (pre-descriptor) stage.
+fn speedName(speed: u32) []const u8 {
+    return switch (speed) {
+        1 => "Full-speed (USB 2.0, 12 Mb/s)",
+        2 => "Low-speed (USB 2.0, 1.5 Mb/s)",
+        3 => "High-speed (USB 2.0, 480 Mb/s)",
+        4 => "SuperSpeed (USB 3.0, 5 Gb/s)",
+        5 => "SuperSpeedPlus (USB 3.1, 10 Gb/s)",
+        else => "unknown speed",
+    };
+}
+
 /// The root-hub port scan: read the capability registers for the port count
 /// and the operational-register offset, then one PORTSC per port. The connect
 /// bit (CCS) and the speed field reflect hardware state directly — no
@@ -129,7 +145,7 @@ fn scanPorts(manager: runtime.ipc.Handle) void {
         if (port_status & 1 == 0) continue; // CCS: nothing connected
         connected += 1;
         const speed = (port_status >> 10) & 0xF; // the PORTSC port-speed class
-        writeLine("usb-xhci-bus: port {d} connected (speed class {d})\n", .{ port, speed });
+        writeLine("usb-xhci-bus: port {d} connected — {s} (speed class {d})\n", .{ port, speedName(speed), speed });
 
         const report = protocol.ChildAdded{
             .parent = controller_id,