Add tactic position generators and tests

fixtures/tactics/back_rank_weakness.json

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+{
+  "patternType": "BACK_RANK_WEAKNESS",
+  "description": "Classic back-rank mate with no escape squares for the king.",
+  "cases": [
+    {
+      "id": "back_rank_rook_mate",
+      "initialFen": "6k1/5ppp/8/8/8/8/5PPP/R5K1 w - - 0 1",
+      "sideToMove": "white",
+      "bestMove": { "san": "Ra8#", "uci": "a1a8" },
+      "resultingFen": "R5k1/5ppp/8/8/8/8/5PPP/6K1 b - - 1 1",
+      "expectedPattern": {
+        "type": "BACK_RANK_WEAKNESS",
+        "side": "white",
+        "attackerSquares": ["a8"],
+        "targetSquares": ["g8"],
+        "explanationKey": "back_rank_mate"
+      },
+      "context": "No luft and the king is boxed in; the rook delivers mate on the back rank.",
+      "tags": ["mate", "fundamental"]
+    }
+  ]
+}

fixtures/tactics/discovered_check.json

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+{
+  "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"]
+    }
+  ]
+}

fixtures/tactics/double_attack.json

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+{
+  "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"]
+    }
+  ]
+}

fixtures/tactics/fork.json

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+{
+  "patternType": "FORK",
+  "description": "Knight fork hitting king and queen simultaneously.",
+  "cases": [
+    {
+      "id": "fork_knight_checks_king_and_queen",
+      "initialFen": "4k3/5q2/8/5N2/8/8/8/6K1 w - - 0 1",
+      "sideToMove": "white",
+      "bestMove": { "san": "Nd6+", "uci": "f5d6" },
+      "resultingFen": "4k3/5q2/3N4/8/8/8/8/6K1 b - - 1 1",
+      "expectedPattern": {
+        "type": "FORK",
+        "side": "white",
+        "attackerSquares": ["d6"],
+        "targetSquares": ["e8", "f7"],
+        "explanationKey": "knight_fork"
+      },
+      "context": "The knight hop gives check on e8 while simultaneously attacking the queen on f7.",
+      "tags": ["tactics", "knight"]
+    }
+  ]
+}

fixtures/tactics/overloading.json

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+{
+  "patternType": "OVERLOADING",
+  "description": "The defending rook on f8 is overloaded, forced to choose between guarding the back rank and protecting f7.",
+  "cases": [
+    {
+      "id": "overloading_rook_on_f8",
+      "initialFen": "5rk1/5p2/8/3Q4/8/8/8/4R1K1 w - - 0 1",
+      "sideToMove": "white",
+      "bestMove": { "san": "Qxf7+", "uci": "d5f7" },
+      "resultingFen": "5rk1/5Q2/8/8/8/8/8/4R1K1 b - - 0 1",
+      "expectedPattern": {
+        "type": "OVERLOADING",
+        "side": "white",
+        "attackerSquares": ["f7"],
+        "targetSquares": ["f8", "f7"],
+        "explanationKey": "back_rank_vs_defense"
+      },
+      "context": "Capturing on f7 lures the only defender; after ...Rxf7 the back rank collapses to Re8#.",
+      "tags": ["overloading", "back_rank_setup"]
+    }
+  ]
+}

fixtures/tactics/pin.json

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+{
+  "patternType": "PIN",
+  "description": "Classical bishop pin on the Ruy Lopez diagonal, tying the c6-knight to the e8-king.",
+  "cases": [
+    {
+      "id": "pin_ruy_lopez_knight_to_king",
+      "initialFen": "r1bqkbnr/pppp1ppp/2n5/4p3/4P3/5N2/PPPP1PPP/RNBQKB1R w KQkq - 2 3",
+      "sideToMove": "white",
+      "bestMove": { "san": "Bb5", "uci": "f1b5" },
+      "resultingFen": "r1bqkbnr/pppp1ppp/2n5/1B2p3/4P3/5N2/PPPP1PPP/RNBQK2R b KQkq - 3 3",
+      "expectedPattern": {
+        "type": "PIN",
+        "side": "white",
+        "attackerSquares": ["b5"],
+        "targetSquares": ["c6"],
+        "keySquares": ["e8"],
+        "explanationKey": "pin_to_king"
+      },
+      "context": "1.e4 e5 2.Nf3 Nc6 3.Bb5 with the knight pinned to the king.",
+      "tags": ["opening", "ruy_lopez"]
+    }
+  ]
+}

fixtures/tactics/skewer.json

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+{
+  "patternType": "SKEWER",
+  "description": "Bishop skewer forcing the king off the diagonal so the trailing queen can be captured.",
+  "cases": [
+    {
+      "id": "skewer_bishop_on_long_diagonal",
+      "initialFen": "7q/6k1/8/8/8/8/1B6/6K1 w - - 0 1",
+      "sideToMove": "white",
+      "bestMove": { "san": "Bc3+", "uci": "b2c3" },
+      "resultingFen": "7q/6k1/8/8/8/2B5/8/6K1 b - - 1 1",
+      "expectedPattern": {
+        "type": "SKEWER",
+        "side": "white",
+        "attackerSquares": ["c3"],
+        "targetSquares": ["g7", "h8"],
+        "keySquares": ["g7"],
+        "explanationKey": "king_in_front_of_queen"
+      },
+      "context": "Minimal material: the move Bc3+ attacks the king on g7; after the king moves the queen on h8 is lost.",
+      "tags": ["endgame", "long_diagonal"]
+    }
+  ]
+}

fixtures/tactics/trapped_piece.json

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+{
+  "patternType": "TRAPPED_PIECE",
+  "description": "Illustrates a bishop trapped in the corner with no safe squares.",
+  "cases": [
+    {
+      "id": "trapped_corner_bishop",
+      "initialFen": "6kb/6P1/5K2/8/8/8/8/8 w - - 0 1",
+      "sideToMove": "white",
+      "bestMove": { "san": "gxh8=Q+", "uci": "g7h8q" },
+      "resultingFen": "6kQ/8/5K2/8/8/8/8/8 b - - 0 1",
+      "expectedPattern": {
+        "type": "TRAPPED_PIECE",
+        "side": "white",
+        "attackerSquares": ["g7"],
+        "targetSquares": ["h8"],
+        "explanationKey": "corner_trap"
+      },
+      "context": "The black bishop on h8 cannot escape because of the pawn on g7 and the king on f6.",
+      "tags": ["endgame", "conversion"]
+    }
+  ]
+}