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/start.zig

@@ -4,8 +4,8 @@
 //! the whole runtime is linked in.
 
 const std = @import("std");
-const system = @import("system.zig");
-const process = @import("process.zig");
+const logging = @import("logging");
+const process = @import("process");
 
 /// The kernel enters at `_start` with rsp 16-aligned, pointing at the System V
 /// process-entry block it built: argc, argv pointers, NULL, envp terminator, the
@@ -31,7 +31,7 @@ export fn rt_start(stack: [*]const u64) callconv(.c) noreturn {
         .count = stack[0],
         .vector = @ptrCast(stack + 1),
     } };
-    system.exit(callMain(init));
+    process.exit(callMain(init));
 }
 
 /// Comptime-dispatch on root.main's signature, in the spirit of std's start.zig:
@@ -68,7 +68,7 @@ fn callMain(init: process.Init) u8 {
             const payload = @call(.auto, root.main, call_arguments) catch |err| {
                 var buffer: [128]u8 = undefined;
                 const line = std.fmt.bufPrint(&buffer, "main returned error: {s}\n", .{@errorName(err)}) catch "main returned an error\n";
-                _ = system.write(line);
+                _ = logging.write(line);
                 return 1; // distinct from panic's 127
             };
             if (@TypeOf(payload) == void) return 0;
@@ -82,6 +82,6 @@ fn callMain(init: process.Init) u8 {
 /// No runtime to unwind into — report a panic as a nonzero exit code.
 pub const panic = std.debug.FullPanic(struct {
     fn panic(_: []const u8, _: ?usize) noreturn {
-        system.exit(127);
+        process.exit(127);
     }
 }.panic);