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.

build.zig

@@ -403,9 +403,6 @@ pub fn build(b: *std.Build) void {
         .root_source_file = b.path("system/services/device-manager/device-manager-protocol.zig"),
     });
     runtime_module.addImport("device-manager-protocol", device_manager_protocol_module);
-    // The USB transfer protocol, so runtime.usb (the class-driver client) can speak
-    // it, the way runtime.input speaks the input protocol.
-    runtime_module.addImport("usb-transfer-protocol", usb_transfer_protocol_module);
     // The block protocol, so runtime.block (the block-device client) can speak it.
     runtime_module.addImport("block-protocol", block_protocol_module);
 
@@ -449,6 +446,20 @@ pub fn build(b: *std.Build) void {
         },
     });
 
+    // The USB class-driver transfer client (library/device/usb/usb.zig): open a device on
+    // the xHCI bus and drive it (control / interrupt / bulk). Bus-family logic a class
+    // driver imports directly — no longer funnelled through runtime. Re-exports usb-abi /
+    // usb-ids as usb.abi / usb.ids for a single USB import.
+    const usb_module = b.addModule("usb", .{
+        .root_source_file = b.path("library/device/usb/usb.zig"),
+        .imports = &.{
+            .{ .name = "runtime", .module = runtime_module },
+            .{ .name = "usb-transfer-protocol", .module = usb_transfer_protocol_module },
+            .{ .name = "usb-abi", .module = usb_abi_module },
+            .{ .name = "usb-ids", .module = usb_ids_module },
+        },
+    });
+
     // Keyboard layouts compiled from the X11 xkeyboard-config database into native Zig
     // (keycode + modifiers -> keysym/character). The `layouts` tables are generated by
     // tools/make-xkeyboard-config.py; `xkeyboard-config` is the hand-written API over them.
@@ -548,12 +559,15 @@ pub fn build(b: *std.Build) void {
     // opens its device through runtime.usb (the transfer protocol) and publishes to
     // the input service. They build chapter-9 class requests from usb-abi.
     const usb_hid_keyboard_exe = addUserBinary(b, kernel_target, runtime_module, mmio_module, xkeyboard_config_module, acpi_ids_module, "usb-hid-keyboard", "system/drivers/usb-hid/keyboard.zig");
+    programModule(usb_hid_keyboard_exe).addImport("usb", usb_module);
     programModule(usb_hid_keyboard_exe).addImport("usb-abi", usb_abi_module);
     const usb_hid_mouse_exe = addUserBinary(b, kernel_target, runtime_module, mmio_module, xkeyboard_config_module, acpi_ids_module, "usb-hid-mouse", "system/drivers/usb-hid/mouse.zig");
+    programModule(usb_hid_mouse_exe).addImport("usb", usb_module);
     programModule(usb_hid_mouse_exe).addImport("usb-abi", usb_abi_module);
     // The USB mass-storage class driver: opens its device via runtime.usb, drives it
     // with Bulk-Only Transport + SCSI, and serves the block protocol under `.block`.
     const usb_storage_exe = addUserBinary(b, kernel_target, runtime_module, mmio_module, xkeyboard_config_module, acpi_ids_module, "usb-storage", "system/drivers/usb-storage/usb-storage.zig");
+    programModule(usb_storage_exe).addImport("usb", usb_module);
     programModule(usb_storage_exe).addImport("block-protocol", block_protocol_module);
     // The FAT filesystem server: mounts the block device and serves it into the VFS
     // at /mnt/usb. Its engine (engine.zig / on-disk.zig) is imported relatively.