Add tactic position generators and tests

fixtures/tactics/double_attack.json

@@ -0,0 +1,22 @@
+{
+  "patternType": "DOUBLE_ATTACK",
+  "description": "Queen move that checks the king while targeting an undefended rook.",
+  "cases": [
+    {
+      "id": "double_attack_queen_check_and_rook",
+      "initialFen": "r3k3/8/8/8/8/8/8/3Q2K1 w - - 0 1",
+      "sideToMove": "white",
+      "bestMove": { "san": "Qa4+", "uci": "d1a4" },
+      "resultingFen": "r3k3/8/8/8/Q7/8/8/6K1 b - - 1 1",
+      "expectedPattern": {
+        "type": "DOUBLE_ATTACK",
+        "side": "white",
+        "attackerSquares": ["a4"],
+        "targetSquares": ["e8", "a8"],
+        "explanationKey": "queen_check_and_rook"
+      },
+      "context": "A single queen move gives check on e8 and simultaneously eyes the rook on a8.",
+      "tags": ["queen", "double_attack"]
+    }
+  ]
+}