usb: a hub yanked from a root port takes its subtree with it (H2)

The hot-plug matrix's predicted bug, found on first contact: tearDownPort
never recursed into a departing hub's children — only tearDownHubDevice
(a hub leaving one level down) did. Yank a populated hub from a root port
and the downstream slots stayed live against vanished hardware, their class
drivers were never reaped, and the replugged hub found its port still
occupied, so nothing ever re-enumerated: the subtree was gone for the boot.

tearDownPort now recurses children-first, exactly like tearDownHubDevice.
The usb-hub-yank case is the discrimination: one device_del removes a hub
carrying a keyboard AND a mouse, both drivers must be reaped, and the
re-added hub must rebind both — it failed against the unfixed bus and
passes with the recursion.

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

@@ -490,6 +490,14 @@ fn deviceIsHub(usb_device: *const library.Device) bool {
 fn tearDownPort(manager: ipc.Handle, engine: *library.Controller, port: u32) void {
     const usb_device = engine.deviceOnPort(port) orelse return;
     std.log.info("port {d} disconnected", .{port});
+    // A hub yanked from a root port takes its whole subtree with it — children
+    // first, recursively, exactly as tearDownHubDevice does for a hub leaving
+    // one level down. Without this, the downstream slots stayed live against
+    // vanished hardware, their class drivers were never reaped, and the
+    // replugged hub found its port still occupied — nothing re-enumerated.
+    if (usb_device.is_hub) {
+        while (engine.nextChildOf(usb_device.slot_id, 0)) |child| tearDownHubDevice(manager, engine, child);
+    }
     for (usb_device.interfaces[0..usb_device.interface_count]) |*interface| {
         if (interface.registered_device_id == 0) continue;
         reportRemoved(manager, (@as(u64, port) << 8) | interface.number, .{ .port = port, .interface = interface.number });

test/qemu_test.py

@@ -1009,6 +1009,34 @@ CASES = [
                r"[\s\S]*device-manager: delegated device \d+ to /system/drivers/usb-hid-keyboard"
                r"[\s\S]*usb-hid-keyboard: ok)",
      "fail":   r"DANOS-TEST-RESULT: FAIL"},
+    # Hot-plug matrix H2 (docs/hot-plug-matrix-plan.md): yank a POPULATED hub.
+    # One device_del removes the hub with a keyboard and a mouse behind it — the
+    # bus's recursive teardown (tearDownHubDevice, children first) must report
+    # every downstream interface removed, the manager must reap BOTH bound
+    # drivers, and re-adding the hub and both devices must rebuild the subtree.
+    # The reaping lines only exist post-yank, so `reaping X ... X: ok` chains
+    # are unambiguously second-generation.
+    {"name": "usb-hub-yank",
+     "build_case": "usb-hid",
+     "smp":    4,
+     "timeout": 150,
+     "qemu_extra": ["-device", "qemu-xhci,id=xhci2",
+                    "-device", "usb-hub,bus=xhci2.0,port=1,id=yhub",
+                    "-device", "usb-kbd,bus=xhci2.0,port=1.1,id=ykbd",
+                    "-device", "usb-mouse,bus=xhci2.0,port=1.2,id=ymouse"],
+     "qmp_sequence": [
+         {"delay": 8,  "command": "device_del", "arguments": {"id": "yhub"}},
+         {"delay": 14, "command": "device_add",
+          "arguments": {"driver": "usb-hub", "bus": "xhci2.0", "port": "1", "id": "yhub2"}},
+         {"delay": 17, "command": "device_add",
+          "arguments": {"driver": "usb-kbd", "bus": "xhci2.0", "port": "1.1", "id": "ykbd2"}},
+         {"delay": 18, "command": "device_add",
+          "arguments": {"driver": "usb-mouse", "bus": "xhci2.0", "port": "1.2", "id": "ymouse2"}},
+     ],
+     "expect": r"(?s)(?=.*hub device slot \d+ disconnected)"
+               r"(?=.*device-manager: reaping \S*usb-hid-keyboard[\s\S]*usb-hid-keyboard: ok)"
+               r"(?=.*device-manager: reaping \S*usb-hid-mouse[\s\S]*usb-hid-mouse: ok)",
+     "fail":   r"DANOS-TEST-RESULT: FAIL"},
     # The kernel VFS root (M-F): the mount table serves the initrd at /system —
     # path resolution, node status/read (an ELF magic), and directory listing,
     # asserted kernel-side.