Phase 2b: wire mkdir + unlink through the VFS to runtime.fs

The engine gained mkdir/unlink in Phase 2a; this exposes them as first-class
filesystem operations so programs can use them.

- vfs protocol: two new path-based operations, mkdir and unlink (appended, so
  existing opcodes/offsets are unchanged).
- VFS router: a forwardPath helper relays a path-based op under a mount to its
  backend; the mkdir/unlink cases forward to the mounted filesystem (the flat
  ramfs refuses them — it has no directories).
- fat server: mkdir -> engine.createDirectory, unlink -> engine.removeFile, each
  resolving the parent via a shared splitParent helper (also used by open-create).
- runtime.fs: makeDirectory(path) and remove(path).
- fat-test now exercises the whole path — mkdir /mnt/usb/TESTDIR, create + write +
  read a file inside it, then remove it — behind a new `fat-mutations` QEMU case.

Verified: zig build, zig build test, zig build check-fat-image, and a sequential
QEMU sweep — fat-mount, fat-mutations (mkdir/write/read/unlink through the mount),
vfs, vfs-client-death, log-flush, orderly-shutdown, initial-ramdisk, smoke — green.

test/qemu_test.py

@@ -320,6 +320,15 @@ CASES = [
      "timeout": 150,
      "expect": r"fat: mounted /mnt/usb[\s\S]*fat-test: ok",
      "fail":   r"DANOS-TEST-RESULT: FAIL"},
+    # Phase 2b: mkdir/unlink through the mount. Reuses the fat-mount build — the
+    # fat-test client, after listing, makes a directory, writes+reads a file inside
+    # it, then removes the file, exercising the whole VFS -> fat mutation path.
+    {"name": "fat-mutations",
+     "build_case": "fat-mount",
+     "smp":    4,
+     "timeout": 150,
+     "expect": r"fat-test: mutations ok",
+     "fail":   r"fat-test: mutations FAILED|fat-test: mkdir .* failed|DANOS-TEST-RESULT: FAIL"},
     # Boot-from-USB smoke: the whole system now boots off the FAT32 image on a
     # usb-storage device (OVMF -> \EFI\BOOT\BOOTX64.efi -> kernel), so the kernel
     # reaching its PASS marker at all proves the USB boot path end to end. Reuses