M9: user-space VFS server + client file API (open/read/write/stat + stdio)

The payoff milestone: files are served by a user-space process, reached over
IPC — the kernel never sees a path or an fd.

- lib/vfs_proto.zig: the VFS wire protocol (Op, Request/Reply fixed header +
  inline payload, Stat), shared by client and server; one message <= MSG_MAX.
- lib/ipc.zig: replyWait() — the server-side dual-return stub (length in rax,
  badge in rdx via a "+{rdx}" read-write operand), deferred from M7.
- sbin/vfs.zig: the real VFS server — an in-heap ramfs (open creates a node)
  with an IPC_ReplyWait dispatch loop serving open/read/write/stat/close.
  Registers its endpoint under the well-known vfs id at startup.
- lib/unistd.zig: POSIX-style client API in rt — a per-process fd table +
  open/close/read/write/lseek/stat, each an IPC_Call to the VFS. The kernel
  knows nothing of fds; the table lives here.
- lib/stdio.zig: C stdio over unistd — FILE + fopen/fclose/fread/fwrite/
  fseek/ftell/rewind/feof/ferror/fputs/fputc/fgetc (unbuffered for now).
- sbin/vfstest.zig: a client that opens/writes/seeks/reads a file and only
  heartbeats "vfstest: ok" if the round trip matched. Packed in the initrd.
- New `vfs` test drives it end to end. initrd test relaxed to generic
  liveness. Suite 31/31.

lib/rt.zig

@@ -14,6 +14,12 @@ pub const sys = @import("sys.zig");
 pub const heap = @import("heap.zig");
 pub const ipc = @import("ipc.zig");
 pub const start = @import("start.zig");
+/// The VFS wire protocol (shared with the VFS server).
+pub const vfsproto = @import("vfs_proto.zig");
+/// POSIX-style file API: open/read/write/lseek/stat/close.
+pub const unistd = @import("unistd.zig");
+/// C stdio: fopen/fread/fwrite/fseek/ftell/fclose over unistd.
+pub const stdio = @import("stdio.zig");
 
 /// Re-exported so a user binary can `pub const panic = rt.panic;`.
 pub const panic = start.panic;