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/fat/fat-test.zig

@@ -5,15 +5,17 @@
 //! kernel test spawns it alongside init.
 
 const std = @import("std");
-const runtime = @import("runtime");
-const fs = runtime.fs;
+const fs = @import("file-system");
+const process = @import("process");
+const time = @import("time");
+const logging = @import("logging");
 
 fn writeLine(comptime fmt: []const u8, arguments: anytype) void {
     var line: [128]u8 = undefined;
-    _ = runtime.system.write(std.fmt.bufPrint(&line, fmt, arguments) catch return);
+    _ = logging.write(std.fmt.bufPrint(&line, fmt, arguments) catch return);
 }
 
-pub fn main(init: runtime.process.Init) void {
+pub fn main(init: process.Init) void {
     _ = init;
 
     // Wait for /mnt/usb to be mounted — the fat server races us at boot (it must
@@ -22,10 +24,10 @@ pub fn main(init: runtime.process.Init) void {
     var tries: u32 = 0;
     while (opened == null and tries < 1400) : (tries += 1) {
         opened = fs.openDirectory("/mnt/usb");
-        if (opened == null) runtime.system.sleep(50);
+        if (opened == null) time.sleepMillis(50);
     }
     var dir = opened orelse {
-        _ = runtime.system.write("fat-test: /mnt/usb never became available\n");
+        _ = logging.write("fat-test: /mnt/usb never became available\n");
         return;
     };
 
@@ -47,7 +49,7 @@ pub fn main(init: runtime.process.Init) void {
         const n = file.read(&magic) orelse 0;
         file.close();
         if (n == 4 and magic[0] == 0x7F and magic[1] == 'E' and magic[2] == 'L' and magic[3] == 'F') {
-            _ = runtime.system.write("fat-test: read /mnt/usb/system/kernel ELF magic ok\n");
+            _ = logging.write("fat-test: read /mnt/usb/system/kernel ELF magic ok\n");
         } else {
             writeLine("fat-test: /mnt/usb/system/kernel read {d} bytes (not ELF magic)\n", .{n});
         }
@@ -68,12 +70,12 @@ pub fn main(init: runtime.process.Init) void {
             writeLine("fat-test: mtime {d}\n", .{attrs.mtime});
             mtime_ok = attrs.mtime > 1_577_836_800; // after 2020-01-01
         }
-        if (mtime_ok) _ = runtime.system.write("fat-test: mtime ok\n");
+        if (mtime_ok) _ = logging.write("fat-test: mtime ok\n");
 
         // Rename it, then read from the new name and confirm the old name is gone.
         const renamed = fs.rename("/mnt/usb/TESTDIR/HELLO.TXT", "/mnt/usb/TESTDIR/RENAMED.TXT");
         const old_gone = !fs.exists("/mnt/usb/TESTDIR/HELLO.TXT");
-        if (renamed and old_gone) _ = runtime.system.write("fat-test: rename ok\n");
+        if (renamed and old_gone) _ = logging.write("fat-test: rename ok\n");
         var readback = false;
         if (fs.open("/mnt/usb/TESTDIR/RENAMED.TXT", .{})) |reopened| {
             var f = reopened;
@@ -85,19 +87,19 @@ pub fn main(init: runtime.process.Init) void {
         const removed = fs.remove("/mnt/usb/TESTDIR/RENAMED.TXT");
         const gone = !fs.exists("/mnt/usb/TESTDIR/RENAMED.TXT");
         if (wrote and mtime_ok and renamed and old_gone and readback and removed and gone) {
-            _ = runtime.system.write("fat-test: mutations ok\n");
+            _ = logging.write("fat-test: mutations ok\n");
         } else {
             writeLine("fat-test: mutations FAILED (wrote={} mtime={} renamed={} oldgone={} read={} removed={} gone={})\n", .{ wrote, mtime_ok, renamed, old_gone, readback, removed, gone });
         }
     } else {
-        _ = runtime.system.write("fat-test: mkdir /mnt/usb/TESTDIR failed\n");
+        _ = logging.write("fat-test: mkdir /mnt/usb/TESTDIR failed\n");
     }
 
     if (count > 0) {
         while (true) {
-            _ = runtime.system.write("fat-test: ok\n");
-            runtime.system.sleep(1000);
+            _ = logging.write("fat-test: ok\n");
+            time.sleepMillis(1000);
         }
     }
-    _ = runtime.system.write("fat-test: root listing was empty\n");
+    _ = logging.write("fat-test: root listing was empty\n");
 }