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.zig

@@ -152,6 +152,10 @@ fn onMessage(message: []const u8, out: []u8, sender: u32, capability: ?runtime.i
     const request = std.mem.bytesToValue(protocol.Request, message[0..protocol.request_size]);
     const payload = message[protocol.request_size..];
 
+    // Stamp create/write with the current wall-clock time (mtime). Cheap, and it
+    // keeps the engine pure (it takes the time as data, not a syscall).
+    filesystem.current_time_epoch = runtime.system.wallClock();
+
     switch (request.operation) {
         .open => return handleOpen(out, payload[0..@min(payload.len, request.len)], request.flags),
         .read => {
@@ -170,7 +174,7 @@ fn onMessage(message: []const u8, out: []u8, sender: u32, capability: ?runtime.i
         .status => {
             const o = openAt(request.node) orelse return fail(out);
             const kind: protocol.NodeKind = if (o.node.is_directory) .directory else .regular;
-            const status = protocol.FileStatus{ .size = o.node.size, .kind = @intFromEnum(kind) };
+            const status = protocol.FileStatus{ .size = o.node.size, .kind = @intFromEnum(kind), .mtime = o.node.mtime };
             return writeReply(out, .{ .status = 0, .len = @sizeOf(protocol.FileStatus) }, std.mem.asBytes(&status));
         },
         .readdir => {