reorg: split the runtime into library/kernel concern modules (C1)

The one giant `runtime` module (with a `system.zig` that was itself a dumping
ground of unrelated syscalls) is split into directly-importable, flat concern
modules under library/kernel/:

  system-call  ipc  memory  process  thread  time  logging  file-system  service  start
  (+ the device/service clients: device, device-manager, block, display, input)

system.zig is dissolved — its functions moved to their concern home (mmap ->
memory, spawn/kill/exit -> process, sleep/clock -> time, write/klog -> logging,
fs* -> file-system). `memory` merges heap+dma+shared-memory behind one flat API
(memory.allocator/dmaAlloc/sharedCreate/mmap), keeping heap's state and malloc
export single. The memory<->thread dependency cycle (heap needs Thread.Mutex,
thread needs mmap) is broken by having thread allocate its own stack via the raw
mmap syscall, so the module graph is a DAG.

This is the atomic step: all 42 internal cross-imports flip from relative to
module imports at once. `runtime.zig` and `system.zig` become thin re-export
SHIMS so the ~38 consumers keep compiling on `runtime.*` untouched; they migrate
to direct imports in C2, after which the shims are deleted (C5).

zig build + zig build test green; 14 QEMU cases pass (smoke, process,
process-kill, thread-spawn/join, logger, vfs, fat-mount, display-native,
usb-storage, virtio-gpu, device-manager, input, power-button).

library/kernel/block.zig

@@ -8,8 +8,8 @@
 //! limit — the same handoff usb-storage uses toward the controller.
 
 const std = @import("std");
-const ipc = @import("ipc.zig");
-const system = @import("system.zig");
+const ipc = @import("ipc");
+const time = @import("time");
 const block_protocol = @import("block-protocol");
 
 pub const Geometry = struct { block_size: u32, block_count: u64 };
@@ -73,7 +73,7 @@ pub fn open() ?Device {
     // not sit a further minute pretending otherwise.
     while (attempts < 600) : (attempts += 1) {
         if (ipc.lookup(.block)) |handle| return .{ .endpoint = handle };
-        system.sleep(50);
+        time.sleepMillis(50);
     }
     return null;
 }