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.

library/device/usb/usb.zig

@@ -16,10 +16,16 @@
 //! the service harness drops buffered-message payloads — see service.zig).
 
 const std = @import("std");
-const ipc = @import("ipc.zig");
+const runtime = @import("runtime");
-const system = @import("system.zig");
+const ipc = runtime.ipc;
+const system = runtime.system;
 const protocol = @import("usb-transfer-protocol");
 
+/// The USB chapter-9 wire ABI and the class taxonomy, re-exported so a class driver reaches
+/// the whole USB domain through its one `usb` import (`usb.abi.getDescriptor`, `usb.ids.Class`).
+pub const abi = @import("usb-abi");
+pub const ids = @import("usb-ids");
+
 pub const Endpoint = protocol.Endpoint;
 pub const InterruptReport = protocol.InterruptReport;
 pub const max_report_data = protocol.max_report_data;

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
+// The USB class-driver client moved to its domain home, library/device/usb (module
-/// (control / interrupt / bulk transfers). See library/runtime/usb.zig.
+// "usb"): it is bus-family logic, not core runtime, and re-exporting it here compiled it
-pub const usb = @import("usb.zig");
+// 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.

system/drivers/usb-hid/keyboard.zig

@@ -4,7 +4,7 @@
 //! keyboard interface (class 3, subclass 1, protocol 1); its assigned device id
 //! arrives as argv[1] and an optional layout name ("us", "gb", ...) as argv[2].
 //! It owns no hardware: it opens its device through the USB transfer protocol
-//! (`runtime.usb`), asks the device for the boot protocol, subscribes to its
+//! (`usb`), asks the device for the boot protocol, subscribes to its
 //! interrupt-IN endpoint, and turns each 8-byte boot report into input-protocol
 //! events, published to the input service — the USB analogue of ps2-bus/keyboard.
 //!
@@ -16,6 +16,7 @@
 
 const std = @import("std");
 const runtime = @import("runtime");
+const usb = @import("usb");
 const usb_abi = @import("usb-abi");
 const xkb = @import("xkeyboard-config");
 const hid = @import("hid-report.zig");
@@ -73,11 +74,11 @@ pub fn main(init: runtime.process.Init) void {
 
     // Hello the manager first (meet the spawn deadline), then open the device.
     if (runtime.device_manager.hello(.device, device_id) == null) return;
-    var device = runtime.usb.open(device_id) orelse {
+    var device = usb.open(device_id) orelse {
         std.log.info("could not open device {d}", .{device_id});
         return;
     };
-    const endpoint = device.findEndpoint(runtime.usb.transfer_type_interrupt, true) orelse {
+    const endpoint = device.findEndpoint(usb.transfer_type_interrupt, true) orelse {
         _ = runtime.system.write("/system/drivers/usb-hid/keyboard: no interrupt-IN endpoint\n");
         return;
     };
@@ -108,9 +109,9 @@ pub fn main(init: runtime.process.Init) void {
             if (signals.has(.terminate)) return;
             continue;
         }
-        if (!got.isMessage() or got.len < @sizeOf(runtime.usb.InterruptReport)) continue;
+        if (!got.isMessage() or got.len < @sizeOf(usb.InterruptReport)) continue;
 
-        const message = std.mem.bytesToValue(runtime.usb.InterruptReport, receive[0..@sizeOf(runtime.usb.InterruptReport)]);
+        const message = std.mem.bytesToValue(usb.InterruptReport, receive[0..@sizeOf(usb.InterruptReport)]);
         if (message.length < @sizeOf(hid.KeyboardReport)) continue;
         const report = std.mem.bytesToValue(hid.KeyboardReport, message.data[0..@sizeOf(hid.KeyboardReport)]);
         const transitions = decoder.feed(report);

system/drivers/usb-hid/mouse.zig

@@ -3,7 +3,7 @@
 //! Spawned by the device manager when the xHCI bus driver reports a HID / boot /
 //! mouse interface (class 3, subclass 1, protocol 2); its assigned device id
 //! arrives as argv[1]. Like the keyboard driver it owns no hardware: it opens its
-//! device through the USB transfer protocol (`runtime.usb`), asks for the boot
+//! device through the USB transfer protocol (`usb`), asks for the boot
 //! protocol, subscribes to its interrupt-IN endpoint, and turns each 3- or 4-byte
 //! boot report into input-protocol mouse events published to the input service.
 //!
@@ -12,6 +12,7 @@
 
 const std = @import("std");
 const runtime = @import("runtime");
+const usb = @import("usb");
 const usb_abi = @import("usb-abi");
 const hid = @import("hid-report.zig");
 const ipc = runtime.ipc;
@@ -38,11 +39,11 @@ pub fn main(init: runtime.process.Init) void {
     };
 
     if (runtime.device_manager.hello(.device, device_id) == null) return;
-    var device = runtime.usb.open(device_id) orelse {
+    var device = usb.open(device_id) orelse {
         std.log.info("could not open device {d}", .{device_id});
         return;
     };
-    const endpoint = device.findEndpoint(runtime.usb.transfer_type_interrupt, true) orelse {
+    const endpoint = device.findEndpoint(usb.transfer_type_interrupt, true) orelse {
         _ = runtime.system.write("/system/drivers/usb-hid/mouse: no interrupt-IN endpoint\n");
         return;
     };
@@ -70,9 +71,9 @@ pub fn main(init: runtime.process.Init) void {
             if (signals.has(.terminate)) return;
             continue;
         }
-        if (!got.isMessage() or got.len < @sizeOf(runtime.usb.InterruptReport)) continue;
+        if (!got.isMessage() or got.len < @sizeOf(usb.InterruptReport)) continue;
 
-        const message = std.mem.bytesToValue(runtime.usb.InterruptReport, receive[0..@sizeOf(runtime.usb.InterruptReport)]);
+        const message = std.mem.bytesToValue(usb.InterruptReport, receive[0..@sizeOf(usb.InterruptReport)]);
         const length = @min(message.length, message.data.len);
         const report = hid.parseMouse(message.data[0..length]) orelse continue;
         const mask = buttonMask(report.buttons);

system/drivers/usb-storage/usb-storage.zig

@@ -3,7 +3,7 @@
 //! Spawned by the device manager when the xHCI bus driver reports a mass-storage
 //! / SCSI / bulk-only interface (class 8, subclass 6, protocol 0x50); its device
 //! id arrives as argv[1]. It owns no hardware: it opens its device through the
-//! USB transfer protocol (`runtime.usb`), then drives it with the BOT command
+//! USB transfer protocol (`usb`), then drives it with the BOT command
 //! cycle — CBW out, an optional data stage, CSW in — carrying SCSI commands
 //! (READ CAPACITY, READ(10), WRITE(10)). Upward it is a block device: it serves
 //! the block protocol under `.block`, the storage a FAT filesystem sits on.
@@ -13,15 +13,16 @@
 
 const std = @import("std");
 const runtime = @import("runtime");
+const usb = @import("usb");
 const scsi = @import("scsi.zig");
 const bot = @import("bulk-only-transport.zig");
 const block_protocol = @import("block-protocol");
 const dma = runtime.dma;
 
 var device_id: u64 = 0;
-var device: runtime.usb.Device = undefined;
+var device: usb.Device = undefined;
-var bulk_in: runtime.usb.Endpoint = undefined;
+var bulk_in: usb.Endpoint = undefined;
-var bulk_out: runtime.usb.Endpoint = undefined;
+var bulk_out: usb.Endpoint = undefined;
 
 // DMA buffers for the transport: the 31-byte CBW, the 13-byte CSW, and a page
 // for the small command data (INQUIRY / READ CAPACITY / the self-check sector).
@@ -70,16 +71,16 @@ var bring_up_failed = false;
 fn initialise(endpoint: runtime.ipc.Handle) bool {
     _ = endpoint;
     if (runtime.device_manager.hello(.device, device_id) == null) return false;
-    device = runtime.usb.open(device_id) orelse {
+    device = usb.open(device_id) orelse {
         std.log.info("could not open device {d}", .{device_id});
         return false;
     };
-    bulk_in = device.findEndpoint(runtime.usb.transfer_type_bulk, true) orelse {
+    bulk_in = device.findEndpoint(usb.transfer_type_bulk, true) orelse {
         _ = runtime.system.write("/system/drivers/usb-storage: no bulk-IN endpoint\n");
         bring_up_failed = true;
         return false;
     };
-    bulk_out = device.findEndpoint(runtime.usb.transfer_type_bulk, false) orelse {
+    bulk_out = device.findEndpoint(usb.transfer_type_bulk, false) orelse {
         _ = runtime.system.write("/system/drivers/usb-storage: no bulk-OUT endpoint\n");
         bring_up_failed = true;
         return false;