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-test.zig

@@ -6,6 +6,7 @@
 
 const std = @import("std");
 const runtime = @import("runtime");
+const fs = runtime.fs;
 
 fn writeLine(comptime fmt: []const u8, arguments: anytype) void {
     var line: [128]u8 = undefined;
@@ -14,38 +15,37 @@ fn writeLine(comptime fmt: []const u8, arguments: anytype) void {
 
 pub fn main(init: runtime.process.Init) void {
     _ = init;
-    const unistd = @import("posix").unistd;
 
     // Wait for /mnt/usb to be mounted — the fat server races us at boot (it must
     // bring up the whole USB storage chain first).
-    var dir: i32 = -1;
+    var opened: ?fs.Directory = null;
     var tries: u32 = 0;
-    while (dir < 0 and tries < 1400) : (tries += 1) {
-        dir = unistd.opendir("/mnt/usb");
-        if (dir < 0) runtime.system.sleep(50);
+    while (opened == null and tries < 1400) : (tries += 1) {
+        opened = fs.openDirectory("/mnt/usb");
+        if (opened == null) runtime.system.sleep(50);
     }
-    if (dir < 0) {
+    var dir = opened orelse {
         _ = runtime.system.write("fat-test: /mnt/usb never became available\n");
         return;
-    }
+    };
 
     var count: u32 = 0;
-    var entry: unistd.DirEntry = .{};
-    while (unistd.readdir(dir, &entry)) {
-        writeLine("fat-test: entry '{s}' kind={d} size={d}\n", .{ entry.name(), entry.kind, entry.size });
+    var entry: fs.Entry = .{};
+    while (dir.next(&entry)) {
+        writeLine("fat-test: entry '{s}' kind={d} size={d}\n", .{ entry.name(), @intFromEnum(entry.kind), entry.size });
         count += 1;
         if (count > 32) break;
     }
-    unistd.closedir(dir);
+    dir.close();
     writeLine("fat-test: listed {d} entries\n", .{count});
 
     // Read a known file off the boot volume through the mount (best effort): the
     // kernel image is an ELF, so its first bytes are the ELF magic.
-    const fd = unistd.open("/mnt/usb/system/kernel", 0);
-    if (fd >= 0) {
+    if (fs.open("/mnt/usb/system/kernel", .{})) |opened_file| {
+        var file = opened_file;
         var magic: [4]u8 = undefined;
-        const n = unistd.read(fd, &magic);
-        unistd.close(fd);
+        const n = file.read(&magic) orelse 0;
+        file.close();
         if (n == 4 and magic[0] == 0x7F and magic[1] == 'E' and magic[2] == 'L' and magic[3] == 'F') {
             _ = runtime.system.write("fat-test: read /mnt/usb/system/kernel ELF magic ok\n");
         } else {