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).

library/device/pci/pci.zig

@@ -7,13 +7,12 @@
 //! The bus enumerator's view — probing arbitrary, not-yet-claimed functions and sizing
 //! their BARs — is a different mechanism and lives in the pci-bus driver. The pure
 //! config-space layout both need (offsets, BAR bit fields) is named once in the `pci-class`
-//! data module; this logic module adds the parts that need `mmio` + `runtime.device`.
+//! data module; this logic module adds the parts that need `mmio` + the `driver` client.
 
 const std = @import("std");
-const runtime = @import("runtime");
 const mmio = @import("mmio");
 const pci_class = @import("pci-class");
-const device = runtime.device;
+const device = @import("driver");
 
 /// A claimed PCI function whose configuration space is mapped (resource 0). `descriptor`
 /// must outlive the Function — the driver's `device.enumerate` buffer does, for the whole

library/device/usb/usb.zig

@@ -16,9 +16,8 @@
 //! the service harness drops buffered-message payloads — see service.zig).
 
 const std = @import("std");
-const runtime = @import("runtime");
-const ipc = runtime.ipc;
-const system = runtime.system;
+const ipc = @import("ipc");
+const time = @import("time");
 const usb_transfer_protocol = @import("usb-transfer-protocol");
 
 /// The USB chapter-9 wire ABI and the class taxonomy, re-exported so a class driver reaches
@@ -125,7 +124,7 @@ pub fn open(device_id: u64) ?Device {
     var attempts: usize = 0;
     const bus = while (attempts < 100) : (attempts += 1) {
         if (ipc.lookup(.usb_bus)) |handle| break handle;
-        system.sleep(20);
+        time.sleepMillis(20);
     } else return null;
 
     const endpoint = ipc.createIpcEndpoint() orelse return null;