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.

library/runtime/fs.zig

@@ -19,7 +19,12 @@ const protocol = @import("vfs-protocol");
 pub const Kind = protocol.NodeKind;
 
 /// A node's metadata (the answer to a status request).
-pub const Attributes = struct { size: u64, kind: Kind };
+pub const Attributes = struct {
+    size: u64,
+    kind: Kind,
+    /// Modification time — Unix epoch seconds, UTC. 0 if the filesystem has none.
+    mtime: u64 = 0,
+};
 
 // Map a wire `NodeKind` value to the enum, defaulting anything unrecognised to
 // `.regular` (the server is trusted, but a value outside the enum would be
@@ -138,7 +143,7 @@ pub const File = struct {
         const r = transact(request, &.{}, &buffer) orelse return null;
         if (r.reply.status != 0 or r.payload.len < @sizeOf(protocol.FileStatus)) return null;
         const status = std.mem.bytesToValue(protocol.FileStatus, buffer[0..@sizeOf(protocol.FileStatus)]);
-        return .{ .size = status.size, .kind = kindFromWire(status.kind) };
+        return .{ .size = status.size, .kind = kindFromWire(status.kind), .mtime = status.mtime };
     }
 
     /// Release the VFS's open handle for this file.