Phase 1a: add the native runtime.fs, retire the posix shim

The first step of the Zig self-hosting roadmap (docs/zig-self-hosting.md): give
danos programs a danos-native file API and remove the premature POSIX compatibility
shim. This also resolves the earlier misplacement of a full-write helper into the
compat layer — that behaviour now lives natively in runtime.fs.File.writeAll.

- library/runtime/fs.zig: the danos-native file client over the VFS (open/read/
  write/writeAll/seekTo/attributes/close, directory listing, mount). Handles are
  *values* — a File/Directory owns its VFS node id and byte offset — so there is no
  per-process fd table or descriptor limit, unlike the POSIX fd model the shim
  emulated. This is where the operations that later become std.os.danos are staged.
- Retire library/posix/ (unistd, stdio): only five call sites used it, all file
  operations, all migrated to runtime.fs — fat (mount), the vfs-test and fat-test
  clients, and init/log-flush (the boot-log flush). stdio was already dead.
- build.zig: drop the posix module, its addUserBinary parameter, the per-binary
  import, and the ~26 call-site arguments.
- Docs: the VFS protocol's client is now runtime.fs; the docs index and
  coding-standards note posix is retired and the foreign-ABI naming exception now
  applies to the future std.os.danos seam; the process-lifecycle note points the
  future musl layer at that same seam rather than the deleted directory.

Deferred by design (see the roadmap): the C-ABI runtime.os errno seam is built at
fork time (its shape must match std/os/danos.zig); truncate/mkdir/rename are
Phase 2; stdio-byte fds and cwd are later slices.

Verified: zig build, zig build test, zig build check-fat-image, and a sequential
QEMU sweep — vfs, vfs-client-death (the park/hold-handle path), fat-mount, log-flush,
orderly-shutdown, initial-ramdisk (log-flush silent in the bare sweep), smoke, init,
usb-storage, device-manager — all green.

system/services/fat/fat.zig

@@ -14,7 +14,6 @@ const runtime = @import("runtime");
 const engine = @import("engine.zig");
 const on_disk = @import("on-disk.zig");
 const protocol = runtime.vfs_protocol;
-const unistd = @import("posix").unistd;
 const dma = runtime.dma;
 
 fn writeLine(comptime fmt: []const u8, arguments: anytype) void {
@@ -106,7 +105,7 @@ fn initialise(endpoint: runtime.ipc.Handle) bool {
     // comes up). From here the VFS routes /mnt/usb/... to this server.
     var tries: u32 = 0;
     while (tries < 100) : (tries += 1) {
-        if (unistd.mount(mount_point, endpoint) == 0) {
+        if (runtime.fs.mount(mount_point, endpoint)) {
             writeLine("/system/services/fat: mounted {s}\n", .{mount_point});
             return true;
         }