USB driver stack: xHCI transfers, HID keyboard/mouse, mass storage

Flesh out the xHCI host-controller driver into a full transfer engine and build
the three USB class drivers on top, all verified end to end under QEMU.

- xHCI engine (usb-xhci-library.zig): controller reset, command/event rings with
  cycle-bit bookkeeping (gated on a No-Op-command proof), device slots, Address
  Device, control transfers, full chapter-9 enumeration, Configure Endpoint, and
  interrupt/bulk transfers. Each interface is device_registered with its
  (class,subclass,protocol) identity, unique per (port,interface).
- Bus<->class transfer protocol (usb-transfer-protocol.zig + runtime.usb): open /
  control / interrupt-subscribe (async report pump on a poll timer) / bulk-by-
  physical-address, so sector data never crosses the 256-byte IPC limit.
- USB HID keyboard + mouse (usb-hid/): decode boot-protocol reports and publish
  to the input service. A USB usage is already the input protocol's keycode.
- USB mass storage (usb-storage/): Bulk-Only Transport + transparent SCSI,
  serving a block device under the new .block service id (block-protocol).
- device-manager matches USB interfaces to class drivers (usbDriverForIdentity).
- usb-abi / usb-ids made importable modules; add HID and mass-storage class
  requests, packTriple, and a usb_device DeviceClass.
- Fix test/qemu_test.py on macOS: the QMP unix-socket path was built from the
  deep worktree path and exceeded the 104-byte sun_path limit, so QEMU exited
  before booting. It now lives under a short temp path.

Tests: usb-report, usb-hid, usb-storage pass under python3 test/qemu_test.py;
host units (usb-abi, usb-ids, hid-report, bulk-only-transport, scsi) green.

system/kernel/tests.zig

@@ -144,6 +144,10 @@ pub fn run(case: []const u8, boot_information: *const BootInformation) void {
         driverRestartTest(boot_information);
     } else if (eql(case, "usb-report")) {
         usbReportTest(boot_information);
+    } else if (eql(case, "usb-hid")) {
+        usbHidTest(boot_information);
+    } else if (eql(case, "usb-storage")) {
+        usbStorageTest(boot_information);
     } else if (eql(case, "device-list")) {
         deviceListTest(boot_information);
     } else if (eql(case, "pci-scan")) {
@@ -1968,6 +1972,40 @@ fn pciScanTest(boot_information: *const BootInformation) void {
 /// the acpi service publishes it; init runs the stop sequence over its children
 /// and asks the power service for S5; the machine powers off (QEMU exits). The
 /// kernel test only spawns init — the ordered chain is the harness assertion.
+/// The USB HID chain, end to end: boot the full service tree (init spawns vfs,
+/// input, device-manager), and let discovery run — the manager matches the PCI
+/// host bridge to pci-bus, pci-bus reports the xHCI controller, usb-xhci-bus
+/// enumerates the HID interfaces, and the manager spawns the class drivers. The
+/// harness's expect regex requires usb-xhci-bus to register the boot-keyboard
+/// interface, the manager to spawn usb-hid-keyboard, and that driver to come up
+/// (open its device, ask for boot protocol, subscribe) — proof the transfer
+/// protocol works class-driver to controller.
+fn usbHidTest(boot_information: *const BootInformation) void {
+    bootServiceTreeTest(boot_information, "usb-hid");
+}
+
+/// The USB storage chain: same full-tree boot, but the harness attaches a
+/// usb-storage device and the expect regex requires usb-storage to come up
+/// (open its device, run the BOT bring-up, read its capacity, and read block 0).
+fn usbStorageTest(boot_information: *const BootInformation) void {
+    bootServiceTreeTest(boot_information, "usb-storage");
+}
+
+fn bootServiceTreeTest(boot_information: *const BootInformation, comptime label: []const u8) void {
+    log("DANOS-TEST-BEGIN: " ++ label ++ "\n", .{});
+    if (boot_information.init_len == 0 or boot_information.initial_ramdisk_len == 0) {
+        check("bootloader handed over init and the initial_ramdisk", false);
+        result();
+        return;
+    }
+    const ramdisk = @as([*]const u8, @ptrFromInt(boot_handoff.physicalToVirtual(boot_information.initial_ramdisk_base)))[0..boot_information.initial_ramdisk_len];
+    process.setInitialRamdisk(ramdisk);
+    const image = @as([*]const u8, @ptrFromInt(boot_handoff.physicalToVirtual(boot_information.init_base)))[0..boot_information.init_len];
+    const spawned = if (process.spawnProcess(image, 4, &.{"/system/services/init"})) true else |_| false;
+    check("init spawned (boots vfs, input, device-manager, and the USB chain)", spawned);
+    result();
+}
+
 fn orderlyShutdownTest(boot_information: *const BootInformation) void {
     log("DANOS-TEST-BEGIN: orderly-shutdown\n", .{});
     if (boot_information.init_len == 0 or boot_information.initial_ramdisk_len == 0) {