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.

system/services/fat/fat-test.zig

@@ -62,6 +62,14 @@ pub fn main(init: runtime.process.Init) void {
             wrote = (f.writeAll("mutation-ok") orelse 0) == "mutation-ok".len;
             f.close();
         }
+        // The created file carries a real modification time (stamped from the RTC).
+        var mtime_ok = false;
+        if (fs.attributes("/mnt/usb/TESTDIR/HELLO.TXT")) |attrs| {
+            writeLine("fat-test: mtime {d}\n", .{attrs.mtime});
+            mtime_ok = attrs.mtime > 1_577_836_800; // after 2020-01-01
+        }
+        if (mtime_ok) _ = runtime.system.write("fat-test: mtime ok\n");
+
         // Rename it, then read from the new name and confirm the old name is gone.
         const renamed = fs.rename("/mnt/usb/TESTDIR/HELLO.TXT", "/mnt/usb/TESTDIR/RENAMED.TXT");
         const old_gone = !fs.exists("/mnt/usb/TESTDIR/HELLO.TXT");
@@ -76,10 +84,10 @@ pub fn main(init: runtime.process.Init) void {
         }
         const removed = fs.remove("/mnt/usb/TESTDIR/RENAMED.TXT");
         const gone = !fs.exists("/mnt/usb/TESTDIR/RENAMED.TXT");
-        if (wrote and renamed and old_gone and readback and removed and gone) {
+        if (wrote and mtime_ok and renamed and old_gone and readback and removed and gone) {
             _ = runtime.system.write("fat-test: mutations ok\n");
         } else {
-            writeLine("fat-test: mutations FAILED (wrote={} renamed={} oldgone={} read={} removed={} gone={})\n", .{ wrote, renamed, old_gone, readback, removed, gone });
+            writeLine("fat-test: mutations FAILED (wrote={} mtime={} renamed={} oldgone={} read={} removed={} gone={})\n", .{ wrote, mtime_ok, renamed, old_gone, readback, removed, gone });
         }
     } else {
         _ = runtime.system.write("fat-test: mkdir /mnt/usb/TESTDIR failed\n");