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/args-echo/args-echo.zig

@@ -8,8 +8,9 @@
 //! proving the kernel-built System V entry stack (argc, argv pointers,
 //! NUL-terminated strings) and the runtime's parsing of it, end to end.
 
-const runtime = @import("runtime");
 
+const process = @import("process");
+const logging = @import("logging");
 /// Recurse with a real frame each level: `depth` levels of ~0.5 KiB, touched
 /// through a volatile pointer so no optimiser can flatten the frames away.
 fn burnStack(depth: usize) u8 {
@@ -21,10 +22,10 @@ fn burnStack(depth: usize) u8 {
     return touch[0] +% burnStack(depth - 1);
 }
 
-pub fn main(init: runtime.process.Init) void {
+pub fn main(init: process.Init) void {
     if (init.arguments.count <= 1) {
         // First instance: spawn the second with real arguments, then exit.
-        _ = runtime.system.spawnWithArguments("args-echo", &.{ "alpha", "beta-42" });
+        _ = process.spawnWithArguments("args-echo", &.{ "alpha", "beta-42" });
         return;
     }
 
@@ -46,5 +47,5 @@ pub fn main(init: runtime.process.Init) void {
     }
     buffer[len] = '\n';
     len += 1;
-    _ = runtime.system.write(buffer[0..len]);
+    _ = logging.write(buffer[0..len]);
 }