Windows driver: queue volume flushes as ordered barriers

Route IRP_MJ_FLUSH_BUFFERS through EncryptedIoQueue for mounted writable non-system volumes. Flushes are represented as zero-length queue items handled by the I/O thread, so ZwFlushBuffersFile runs after earlier encrypted write fragments before completing to the caller.

Also perform a best-effort ZwFlushBuffersFile before closing writable mounted-volume host handles, after the encrypted I/O queue has drained, so clean dismount/shutdown paths push the host file or raw device before close.

This keeps the change focused on ordinary mounted-volume flush ordering and avoids system-encryption, boot-drive, and header-update paths.

src/Driver/EncryptedIoQueue.h

@@ -155,6 +155,7 @@ typedef struct
 	EncryptedIoQueue *Queue;
 	PIRP OriginalIrp;
 	BOOL Write;
+	BOOL Flush;
 	ULONG OriginalLength;
 	LARGE_INTEGER OriginalOffset;
 	NTSTATUS Status;