Phase 2d (ii): filesystem modification time (mtime)

Completes Phase 2: the FAT filesystem now stamps and reports a real modification
time, built on the Phase 2d(i) kernel wall-clock. This is the last stat field the
compiler's build cache needs to reason about (source vs cached output).

- on-disk.zig: fatToEpoch / epochToFatDateTime convert between the two 16-bit DOS
  date/time fields and Unix epoch seconds (UTC — FAT has no timezone). Host-tested
  round-trip + an absolute check (1577836800 == 2020-01-01).
- engine: a settable current_time_epoch that create/write stamp into the entry's
  write (and creation) date/time; Node/Listing gained an mtime decoded from those
  fields on read. Host test: a create stamps the mtime, read back through resolve
  and listEntry.
- vfs protocol FileStatus + runtime.fs.Attributes gained an mtime field; the fat
  server sets current_time_epoch from runtime.system.wallClock() per request and
  returns mtime from stat. The flat ramfs reports 0 (it has no timestamps).
- fat-test reads the created file's mtime through stat and checks it is a real
  current time, behind a new `fat-mtime` QEMU case.

Verified against the host: the guest stamped mtime 1783971676 while the host clock
was 1783971680 (boot+test lag) — the file's mtime is real current time. zig build,
zig build test (the epoch<->DOS conversions + the engine mtime test),
zig build check-fat-image, and a sequential QEMU sweep — fat-mount, fat-mutations,
fat-rename, fat-mtime, vfs, vfs-client-death, log-flush, orderly-shutdown,
initial-ramdisk, smoke, wall-clock, usb-storage — all green. mode/inode remain.

test/qemu_test.py

@@ -342,6 +342,14 @@ CASES = [
      "timeout": 150,
      "expect": r"fat-test: rename ok",
      "fail":   r"fat-test: mutations FAILED|DANOS-TEST-RESULT: FAIL"},
+    # Phase 2d: filesystem timestamps — a freshly-created file's mtime is a real
+    # current wall-clock time (stamped from the RTC), read back through stat.
+    {"name": "fat-mtime",
+     "build_case": "fat-mount",
+     "smp":    4,
+     "timeout": 150,
+     "expect": r"fat-test: mtime 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