C2: migrate consumers off the runtime shim to direct concern-module imports

Every user binary and the two device-logic library modules (pci, usb) now
`@import` the concern modules directly instead of aliasing through `runtime`:

  runtime.ipc/process/time/service/input/block/display  -> @import("<module>")
  runtime.device / runtime.device_manager               -> @import("driver")
  runtime.fs                                             -> @import("file-system")
  runtime.Thread                                         -> @import("thread").Thread
  runtime.system.{write,writeRecord,klog*}              -> logging.*
  runtime.system.{sleep,timerOnce,wallClock,clock}     -> time.*
  runtime.system.{spawn*,kill,exit,yield,processes,...}-> process.*
  runtime.system.{mmap,munmap,PROT_*}                  -> memory.*
  runtime.dma.* / runtime.shared_memory.* / runtime.allocator -> memory.*

Each consumer keeps its own alias name (e.g. `const device = @import("driver")`),
so call sites are unchanged and there are no collisions with local `driver`
variables. build.zig now injects the concern modules into every user binary via
`default_imports`; pci/usb module import lists were updated to match.

The `runtime` and `system` shims remain for one more step (root.zig still uses
runtime); they are deleted in C5. Nothing but root.zig imports `runtime` now.

Verified: zig build, zig build test, and 17 QEMU cases (smoke, device-manager,
logger, fat-mount, fat-mutations, usb-storage, usb-hid, display-native,
virtio-gpu, input, thread-spawn, thread-mutex, process-kill, shared-memory,
driver-restart, acpi-ps2, pci-scan).

system/drivers/ps2-bus/keyboard.zig

@@ -15,12 +15,16 @@
 //!                                      -> xkeyboard-config -> character -> key_press
 
 const std = @import("std");
-const runtime = @import("runtime");
+const device = @import("driver");
+const ipc = @import("ipc");
+const process = @import("process");
+const time = @import("time");
+const input = @import("input");
+const memory = @import("memory");
+const logging = @import("logging");
 const xkb = @import("xkeyboard-config");
 const ps2 = @import("ps2-library.zig");
 const scancode = @import("scancode.zig");
-const device = runtime.device;
-const ipc = runtime.ipc;
 const input_protocol = @import("input-protocol");
 
 /// Look up the ps2-bus service, retrying while the bus (which spawned us before
@@ -29,7 +33,7 @@ fn lookupBus() ?ipc.Handle {
     var attempts: usize = 0;
     while (attempts < 100) : (attempts += 1) {
         if (ipc.lookup(.ps2_bus)) |handle| return handle;
-        runtime.system.sleep(50);
+        time.sleepMillis(50);
     }
     return null;
 }
@@ -64,16 +68,16 @@ fn modifierWord(modifiers: scancode.ModifierSnapshot) u32 {
     return word;
 }
 
-pub fn main(init: runtime.process.Init) void {
+pub fn main(init: process.Init) void {
     const hid = init.arguments.get(1).?;
     if (hid.len == 0) {
-        _ = runtime.system.write("/system/drivers/ps2-bus/keyboard: no HID argument\n");
+        _ = logging.write("/system/drivers/ps2-bus/keyboard: no HID argument\n");
         return;
     }
     std.log.info("starting for hid {s}", .{hid});
 
-    const buffer = runtime.allocator().alloc(device.DeviceDescriptor, 64) catch {
-        _ = runtime.system.write("/system/drivers/ps2-bus/keyboard: out of memory\n");
+    const buffer = memory.allocator().alloc(device.DeviceDescriptor, 64) catch {
+        _ = logging.write("/system/drivers/ps2-bus/keyboard: out of memory\n");
         return;
     };
     if (device.findDeviceDescriptorByHid(buffer, hid) == null) {
@@ -90,33 +94,33 @@ pub fn main(init: runtime.process.Init) void {
     // Attach to the bus: hand it our endpoint, and it forwards every byte the
     // keyboard sends (it owns the controller; we own the decoding).
     const bus = lookupBus() orelse {
-        _ = runtime.system.write("/system/drivers/ps2-bus/keyboard: ps2-bus service unavailable\n");
+        _ = logging.write("/system/drivers/ps2-bus/keyboard: ps2-bus service unavailable\n");
         return;
     };
     const endpoint = ipc.createIpcEndpoint() orelse {
-        _ = runtime.system.write("/system/drivers/ps2-bus/keyboard: no endpoint\n");
+        _ = logging.write("/system/drivers/ps2-bus/keyboard: no endpoint\n");
         return;
     };
     var attach = ps2.AttachRequest{ .device_type = @intFromEnum(ps2.DeviceType.keyboard) };
     var attach_reply: [@sizeOf(ps2.AttachReply)]u8 = undefined;
     const attached = ipc.callCap(bus, std.mem.asBytes(&attach), &attach_reply, endpoint) catch {
-        _ = runtime.system.write("/system/drivers/ps2-bus/keyboard: attach call failed\n");
+        _ = logging.write("/system/drivers/ps2-bus/keyboard: attach call failed\n");
         return;
     };
     if (attached.len < @sizeOf(ps2.AttachReply) or
         std.mem.bytesToValue(ps2.AttachReply, attach_reply[0..@sizeOf(ps2.AttachReply)]).status != @intFromEnum(ps2.AttachStatus.ok))
     {
-        _ = runtime.system.write("/system/drivers/ps2-bus/keyboard: attach refused\n");
+        _ = logging.write("/system/drivers/ps2-bus/keyboard: attach refused\n");
         return;
     }
 
     // Broadcast keyboard events through the input service so programs can listen
     // for them (docs/input.md).
-    var source = runtime.input.connectSource() orelse {
-        _ = runtime.system.write("/system/drivers/ps2-bus/keyboard: input service unavailable\n");
+    var source = input.connectSource() orelse {
+        _ = logging.write("/system/drivers/ps2-bus/keyboard: input service unavailable\n");
         return;
     };
-    _ = runtime.system.write("/system/drivers/ps2-bus/keyboard: ok\n");
+    _ = logging.write("/system/drivers/ps2-bus/keyboard: ok\n");
 
     var decoder = scancode.Decoder{};
     var state = scancode.KeyboardState{};