Phase 2c: rename, wired through the VFS to runtime.fs

Completes the Phase 2 FAT mutation set (truncate, mkdir, unlink, rename).

- engine: rename(dir, old_name, new_name) rewrites an existing entry's 8.3 name in
  place within the same directory. Refuses a missing source, a non-8.3 target, or a
  name that already exists; drops any long-name entries on the old file (it takes
  its new 8.3 name), LFN-aware like removeFile. Cross-directory and long-name-
  preserving rename are noted limitations. Host-tested (rename keeps contents;
  collision, non-8.3, and missing-source are refused).
- vfs protocol: a `rename` operation whose payload is old-path, a 0x00 separator,
  then new-path.
- VFS router: a forwardRename helper + a `.rename` case that requires both paths
  under the same mount (cross-filesystem rename is refused) and forwards the
  mount-relative old+new.
- fat server: a `.rename` handler that requires the same parent directory and calls
  engine.rename.
- runtime.fs: rename(old_path, new_path).
- fat-test now renames the file it created (before removing it) and asserts the old
  name is gone, behind a new `fat-rename` QEMU case.

Verified: zig build, zig build test (the engine rename unit test), zig build
check-fat-image, and a sequential QEMU sweep — fat-mount, fat-mutations, fat-rename,
vfs, vfs-client-death, log-flush, orderly-shutdown, initial-ramdisk, smoke — green.

test/qemu_test.py

@@ -329,6 +329,14 @@ CASES = [
      "timeout": 150,
      "expect": r"fat-test: mutations ok",
      "fail":   r"fat-test: mutations FAILED|fat-test: mkdir .* failed|DANOS-TEST-RESULT: FAIL"},
+    # Phase 2c: rename through the mount — fat-test renames the file it created
+    # before removing it, and confirms the old name is gone.
+    {"name": "fat-rename",
+     "build_case": "fat-mount",
+     "smp":    4,
+     "timeout": 150,
+     "expect": r"fat-test: rename ok",
+     "fail":   r"fat-test: mutations 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