Add tactic position generators and tests

fixtures/tactics/discovered_check.json

@@ -0,0 +1,23 @@
+{
+  "patternType": "DISCOVERED_CHECK",
+  "description": "Bishop steps aside to reveal a rook check along the open file.",
+  "cases": [
+    {
+      "id": "discovered_check_rook_on_open_file",
+      "initialFen": "4k3/8/8/8/8/8/4B3/4R1K1 w - - 0 1",
+      "sideToMove": "white",
+      "bestMove": { "san": "Bb5+", "uci": "e2b5" },
+      "resultingFen": "4k3/8/8/1B6/8/8/8/4R1K1 b - - 1 1",
+      "expectedPattern": {
+        "type": "DISCOVERED_CHECK",
+        "side": "white",
+        "attackerSquares": ["e1"],
+        "targetSquares": ["e8"],
+        "keySquares": ["b5"],
+        "explanationKey": "rook_file_discovery"
+      },
+      "context": "Moving the bishop from e2 to b5 exposes the rook on e1, delivering check on the e-file.",
+      "tags": ["discovery", "rook_file"]
+    }
+  ]
+}