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/display/display.zig

@@ -12,13 +12,13 @@
 //!
 //!  It takes over the framebuffer feature that was setup during system boot.
 const std = @import("std");
-const runtime = @import("runtime");
+const device = @import("driver");
+const ipc = @import("ipc");
+const process = @import("process");
+const service = @import("service");
+const device_manager = @import("driver");
+const logging = @import("logging");
 const mmio = @import("mmio");
-const device = runtime.device;
-const dma = runtime.dma;
-const shared_memory = runtime.shared_memory;
-const system = runtime.system;
-const ipc = runtime.ipc;
 const display_protocol = @import("display-protocol");
 const scanout_protocol = @import("scanout-protocol");
 var device_id: u64 = 0;
@@ -29,7 +29,7 @@ fn initialise(endpoint: ipc.Handle) bool {
     // and serve its display engine; we report no children). Best-effort: without a
     // manager the driver still runs standalone; when present, the manager marks us
     // up before the hello deadline and restarts us if we die.
-    _ = runtime.device_manager.hello(.device, device_id);
+    _ = device_manager.hello(.device, device_id);
     return true;
 }
 
@@ -42,16 +42,16 @@ fn onMessage(message: []const u8, reply: []u8, sender: u32, capability: ?ipc.Han
     return 0;
 }
 
-pub fn main(init: runtime.process.Init) void {
+pub fn main(init: process.Init) void {
     const argument = init.arguments.get(1) orelse {
-        _ = system.write("display: missing device id (argv[1])\n");
+        _ = logging.write("display: missing device id (argv[1])\n");
         return;
     };
     device_id = std.fmt.parseInt(u64, argument, 10) catch {
         std.log.info("malformed device id '{s}'", .{argument});
         return;
     };
-    runtime.service.run(256, .{
+    service.run(256, .{
         .service = .scanout,
         .init = initialise,
         .on_message = onMessage,