Basic sketch for state snapshots: an extra field on Target.

I think it doesn't make sense for states to own a target as that complicates the concept of Media. Plus they're distinct because it makes sense to have only one per Target. Let's see how this pans out.

Storage/State/SNA.cpp

@@ -0,0 +1,36 @@
+//
+//  SNA.cpp
+//  Clock Signal
+//
+//  Created by Thomas Harte on 24/04/2021.
+//  Copyright © 2021 Thomas Harte. All rights reserved.
+//
+
+#include "SNA.hpp"
+
+using namespace Storage::State;
+
+std::unique_ptr<Analyser::Static::Target> SNA::load(const std::string &file_name) {
+	// 0x1a byte header:
+	//
+	//	00	I
+	//	01	HL'
+	//	03	DE'
+	//	05	BC'
+	//	07	AF'
+	//	09	HL
+	//	0B	DE
+	//	0D	BC
+	//	0F	IY
+	//	11	IX
+	//	13	IFF2 (in bit 2)
+	//	14	R
+	//	15	AF
+	//	17	SP
+	//	19	interrupt mode
+	//	1A	border colour
+	//	1B–	48kb RAM contents
+
+	(void)file_name;
+	return nullptr;
+}