reorg: move the USB client to library/device/usb (drop runtime.usb)

The USB class-driver transfer client was library/runtime/usb.zig, re-exported
as runtime.usb — which compiled the USB client and usb-transfer-protocol into
every user binary (init, fat, the compositor…), none of which speak USB. It is
bus-family logic, not core runtime.

Move it to its domain home, library/device/usb/usb.zig (module "usb"), alongside
the usb-abi and usb-ids data modules; fix its internal imports to go through the
runtime module; and re-export usb.abi / usb.ids so a class driver reaches the
whole USB domain through one import. runtime.usb and runtime's
usb-transfer-protocol import are removed; the three class drivers
(usb-hid keyboard/mouse, usb-storage) import module "usb" directly.

zig build + test green; usb-hid, usb-storage pass.

library/runtime/runtime.zig

@@ -46,9 +46,9 @@ pub const dma = @import("dma.zig");
 /// and docs/display-v2.md.
 pub const shared_memory = @import("shared-memory.zig");
 
-/// USB class-driver client: open a device on the xHCI bus and drive it
-/// (control / interrupt / bulk transfers). See library/runtime/usb.zig.
-pub const usb = @import("usb.zig");
+// The USB class-driver client moved to its domain home, library/device/usb (module
+// "usb"): it is bus-family logic, not core runtime, and re-exporting it here compiled it
+// into every binary. USB class drivers import it directly with @import("usb").
 
 /// Block-device client: read/write a block device (a USB stick, via
 /// usb-storage). See library/runtime/block.zig.