usb: [diagnostic] dump all root-port PORTSC + log port-change events

Branch-only debugging for the real SuperSpeed compound hub: the spin fix
(27f87cb) stopped the hang — the SS hub's 4 SuperSpeed ports now enumerate
(all empty, correct: the full-speed keyboard isn't a SuperSpeed device) —
but the USB2 COMPANION hub, where the keyboard actually lives, never
appears. Only root ports 19 (SS hub) and 21 (SS storage) connect.

Dump every root port's raw PORTSC at scan (connect/enable/link-state/
speed) and log every root port-change event, so the next boot shows
whether the companion is connected on a USB2 port we misread, connects
late as a port-change event, or is simply absent. Diagnostic logging —
to be removed once the companion path works.

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

@@ -196,6 +196,18 @@ fn scanPorts(manager: runtime.ipc.Handle) void {
     };
     std.log.info("{d} root-hub ports", .{engine.max_ports});
 
+    // Diagnostic: dump every root port's raw PORTSC (CCS = connect, PED =
+    // enabled, PLS = link state, speed) so a USB2 companion hub that isn't
+    // enumerating is visible — is it connected on a port we misread, or absent?
+    {
+        var p: u32 = 1;
+        while (p <= engine.max_ports) : (p += 1) {
+            const portsc = engine.portStatus(p);
+            if (portsc & 1 != 0 or (portsc >> 5) & 0xF != 5) // connected, or not in the disabled/disconnected link state
+                std.log.info("PORTSC[{d}] = 0x{x:0>8} ccs={d} ped={d} pls={d} speed={d}", .{ p, portsc, portsc & 1, (portsc >> 1) & 1, (portsc >> 5) & 0xF, (portsc >> 10) & 0xF });
+        }
+    }
+
     var port: u32 = 1;
     var connected: u32 = 0;
     while (port <= engine.max_ports) : (port += 1) {
@@ -493,7 +505,9 @@ fn onNotification(badge: u64) void {
         engine.pump();
         while (engine.takePortChange()) |port| {
             const manager = manager_handle orelse break;
-            if (engine.portConnected(port)) {
+            const connected = engine.portConnected(port);
+            std.log.info("root port {d} change: {s} (PORTSC 0x{x:0>8})", .{ port, if (connected) "connected" else "empty", engine.portStatus(port) });
+            if (connected) {
                 if (engine.deviceOnPort(port) == null) bringUpPort(manager, engine, port);
             } else {
                 tearDownPort(manager, engine, port);