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/parameters.zig

@@ -17,10 +17,13 @@ pub const maximum_cpus = 128;
 
 /// Maximum tasks (kernel threads) alive at once — the static task-table size. Each
 /// online core consumes one slot for its idle task, plus task 0 on the BSP. Sized
-/// for the initial-ramdisk sweep (15 bundled binaries spawned at once) plus the
+/// for the initial-ramdisk sweep (the bundled binaries spawned at once) plus the
 /// device manager's supervised children with room to grow — at 16 the sweep
-/// started failing spawns once the bundle passed a dozen binaries.
-pub const maximum_tasks = 32;
+/// started failing spawns once the bundle passed a dozen binaries. Raised to 48
+/// for the USB stack: the xHCI bus driver spawns a supervised class-driver instance
+/// per matched interface (keyboard, mouse, mass storage), on top of the FAT and
+/// block servers and the growing ramdisk bundle.
+pub const maximum_tasks = 48;
 
 /// Each task's kernel stack (also each AP's bring-up stack), in bytes.
 pub const kernel_stack_size = 16 * 1024;