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/services/input-test/input-test.zig

@@ -7,21 +7,20 @@
 //! device classes to one subscription — source -> service -> subscriber, per device.
 
 const std = @import("std");
-const runtime = @import("runtime");
-const input = runtime.input;
-const system = runtime.system;
+const input = @import("input");
+const logging = @import("logging");
 
 fn writeLine(comptime fmt: []const u8, arguments: anytype) void {
     var line: [128]u8 = undefined;
-    _ = system.write(std.fmt.bufPrint(&line, fmt, arguments) catch return);
+    _ = logging.write(std.fmt.bufPrint(&line, fmt, arguments) catch return);
 }
 
 pub fn main() void {
     var listener = input.subscribeAll() orelse {
-        _ = system.write("input-test: could not subscribe\n");
+        _ = logging.write("input-test: could not subscribe\n");
         return;
     };
-    _ = system.write("input-test: subscribed\n");
+    _ = logging.write("input-test: subscribed\n");
 
     var seen_keyboard = false;
     var seen_mouse = false;
@@ -45,7 +44,7 @@ pub fn main() void {
         // The success marker: only once every class has been routed here does this appear,
         // and then it heartbeats. Seeing "input-test: ok" proves per-device fan-out works.
         if (seen_keyboard and seen_mouse and seen_joystick) {
-            _ = system.write("input-test: ok all classes received (keyboard, mouse, joystick)\n");
+            _ = logging.write("input-test: ok all classes received (keyboard, mouse, joystick)\n");
         }
     }
 }