fat/usb-storage: flush the device cache on close so writes survive power-off

init writes /mnt/usb/DANOS.LOG at shutdown and then enters S5 — but the write sat in the USB flash controller's write cache and was lost when power was cut, because nothing issued SCSI SYNCHRONIZE CACHE. Invisible in QEMU (its backing file commits immediately); real on hardware. The boot-time flush survived only because the machine kept running afterward and the cache drained on its own.

- block protocol gains a flush op; usb-storage serves it with SYNCHRONIZE CACHE (10); runtime.block gains Device.flush()
- the FAT server tracks whether blocks were written and, on a file close, commits the device cache (durable-on-close — the right default for removable media, and it makes init's existing shutdown close() persist the log before S5, no init/VFS change needed)
- verified the SYNCHRONIZE CACHE actually reaches the driver on each dirty close; usb-storage/fat-mount/mutations/rename/mtime/log-flush all green

library/runtime/block.zig

@@ -38,6 +38,13 @@ pub const Device = struct {
         return self.transfer(.write, lba, count, physical);
     }
 
+    /// Commit any device write cache to stable media (SCSI SYNCHRONIZE CACHE), so
+    /// prior writes survive a power-off. A filesystem calls this before the machine
+    /// goes down; no data transfer, so the buffer arguments are unused.
+    pub fn flush(self: Device) bool {
+        return self.transfer(.flush, 0, 0, 0);
+    }
+
     fn transfer(self: Device, operation: protocol.Operation, lba: u64, count: u32, physical: u64) bool {
         var request = protocol.Request{ .operation = @intFromEnum(operation), .lba = lba, .count = count, .physical = physical };
         var reply: [protocol.reply_size]u8 = undefined;