reorg: begin library/protocol/ — move block + usb-transfer

Start the protocol tree: wire protocols move to library/protocol/<name>/,
entry file <name>-protocol.zig, module name unchanged. block and usb-transfer
are pure moves — every consumer already imports them by module name, so only
the b.addModule paths change and the empty system/services/block/ is removed.

  block-protocol         -> library/protocol/block/block-protocol.zig
  usb-transfer-protocol  -> library/protocol/usb-transfer/usb-transfer-protocol.zig

zig build + test green.

system/services/device-manager/device-manager.zig

@@ -42,6 +42,13 @@ const virtio_gpu_pci_class: u64 = pci_class.ClassCode.pack(.{
     .prog_if = 0,
 });
 
+
+const vga_compatible_gpu_pci_class: u64 = pci_class.ClassCode.pack(.{
+    .base = @intFromEnum(pci_class.BaseClass.display),
+    .subclass = @intFromEnum(pci_class.display.SubClass.vga_compatible),
+    .prog_if = 0,
+});
+
 /// The driver that serves a *reported* PCI function (M19.3: matching moved
 /// from the boot snapshot to the bus reports), or null. A machine can carry
 /// several identical controllers — one driver instance per reported device,
@@ -49,6 +56,7 @@ const virtio_gpu_pci_class: u64 = pci_class.ClassCode.pack(.{
 fn pciDriverForIdentity(identity: u64) ?[]const u8 {
     return switch (identity) {
         xhci_pci_class => "/system/drivers/usb-xhci-bus",
+        vga_compatible_gpu_pci_class => "/system/drivers/display",
         virtio_gpu_pci_class => "/system/drivers/virtio-gpu",
         else => null,
     };