Post-reorg cleanup: POSIX layer, and naming fixes

Follow-up to the monorepo re-org. Suite 35/35 plus host tests green.

POSIX compatibility is now its own library, library/posix/ (unistd, stdio),
layered strictly over the runtime — it calls the runtime's IPC/heap, never
system calls directly. The runtime is now POSIX-free (the danos-native
application ABI). The VFS wire protocol is danos-native throughout
(Stat -> FileStatus, .stat -> .status, O_CREAT -> create); the POSIX layer
maps the POSIX spellings at the boundary. The coding standard's ABI-name
exception is scoped to one place: a file is allowed POSIX spellings only if it
lives under library/posix/ — everywhere else, danos naming with no exception.

Naming fixes, all mechanical:
- initrd -> initial-ramdisk: the source file, the module, the tool
  (make-initial-ramdisk.py), the artifact (initial-ramdisk.img, including the
  bootloader's load path), and the identifiers.
- system/kernel/device-service.zig -> devices-broker.zig: it is ring-0 kernel
  code (the trusted device table + claim capability), not a ring-3 service. The
  future user-space device *manager* (policy) will live in system/services/.
- Dropped the daemon `d` suffix: hpetd -> hpet, busd -> bus. A driver lives in
  system/drivers/, so the folder already says what it is; encoding the role in
  the name too is redundant. The coding standard drops that exception.
- system/devices/aml/interp.zig -> interpreter.zig (the type was already
  Interpreter).

system/services/vfs/vfs-test.zig

@@ -1,13 +1,13 @@
 //! /sbin/vfstest — a client that proves the VFS round trip end to end: open a
 //! file through the `runtime` file API, write to it, seek back, read it, and compare.
 //! On success it heartbeats "vfstest: ok" so the kernel test can observe it;
-//! on failure it reports what went wrong. Shipped in the initrd alongside vfs.
+//! on failure it reports what went wrong. Shipped in the initial_ramdisk alongside vfs.
 
 const std = @import("std");
 const runtime = @import("runtime");
 
 pub fn main() void {
-    const u = runtime.unistd;
+    const u = @import("posix").unistd;
     const payload = "hello-vfs";
 
     // The VFS server may not have registered yet — retry open until it's up.