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-source/input-source.zig

@@ -11,13 +11,14 @@
 //! decoded hardware is a follow-up (see docs/input.md).
 
 const std = @import("std");
-const runtime = @import("runtime");
-const input = runtime.input;
-const system = runtime.system;
+const input = @import("input");
+const process = @import("process");
+const time = @import("time");
+const logging = @import("logging");
 
-pub fn main(init: runtime.process.Init) void {
+pub fn main(init: process.Init) void {
     var source = input.connectSource() orelse {
-        _ = system.write("input-source: input service unavailable\n");
+        _ = logging.write("input-source: input service unavailable\n");
         return;
     };
 
@@ -26,7 +27,7 @@ pub fn main(init: runtime.process.Init) void {
     // classes to exercise the service's per-device routing (the `input` test).
     const mode = init.arguments.get(1) orelse "rotate";
     if (std.mem.eql(u8, mode, "mouse")) {
-        _ = system.write("input-source: publishing synthetic mouse motion\n");
+        _ = logging.write("input-source: publishing synthetic mouse motion\n");
         while (true) {
             _ = source.publishMouseEvent(.{
                 .kind = @intFromEnum(input.MouseEventKind.motion),
@@ -37,11 +38,11 @@ pub fn main(init: runtime.process.Init) void {
                 .scroll_y = 0,
                 .buttons = 0,
             });
-            system.sleep(20); // ~50 events/sec: moves the cursor briskly
+            time.sleepMillis(20); // ~50 events/sec: moves the cursor briskly
         }
     }
 
-    _ = system.write("input-source: publishing synthetic input events\n");
+    _ = logging.write("input-source: publishing synthetic input events\n");
     var step: usize = 0;
     while (true) : (step +%= 1) {
         // Rotate across the device classes so every publish path (and the service's
@@ -51,6 +52,6 @@ pub fn main(init: runtime.process.Init) void {
             1 => _ = source.publishMouseEvent(input.syntheticMouseEvent(step)),
             else => _ = source.publishJoystickEvent(input.syntheticJoystickEvent(step)),
         }
-        system.sleep(200);
+        time.sleepMillis(200);
     }
 }