Add Index access methods and validation functions to PIVector2D

- Add operator[], element(), and at() overloads for Index struct
- Add isValid() and isNotValid() methods to Index
- Add default initialization to -1 (invalid state)
- Add parameterized constructor Index(row, col)
- Add tests for new functionality

libs/main/containers/pivector2d.h

@@ -55,8 +55,14 @@ public:
 	//! \~english Index structure for 2D array elements (row, column).
 	//! \~russian Структура индекса для элементов двумерного массива (строка, столбец).
 	struct Index {
-		ssize_t row;
-		ssize_t col;
+		ssize_t row    = -1;
+		ssize_t col    = -1;
+
+		inline Index() = default;
+		inline Index(ssize_t r, ssize_t c): row(r), col(c) {}
+
+		inline bool isValid() const { return row >= 0 && col >= 0; }
+		inline bool isNotValid() const { return !isValid(); }
 	};
 
 	//! \~english Constructs an empty 2D array. No memory is allocated.

tests/math/testpivector2d.cpp

@@ -1489,3 +1489,36 @@ TEST_F(Vector2DTest, index_structure_with_brace_initialization_works) {
 	vec[{static_cast<ssize_t>(ROWS_COUNT_INIT - 1), static_cast<ssize_t>(COLS_COUNT_INIT - 1)}] = 200;
 	EXPECT_EQ(vec.element(ROWS_COUNT_INIT - 1, COLS_COUNT_INIT - 1), 200);
 }
+
+TEST(Vector2DIndexTest, default_constructor_initializes_to_invalid) {
+	PIVector2D<int>::Index idx;
+	EXPECT_TRUE(idx.isNotValid());
+	EXPECT_FALSE(idx.isValid());
+	EXPECT_EQ(idx.row, -1);
+	EXPECT_EQ(idx.col, -1);
+}
+
+TEST(Vector2DIndexTest, parameterized_constructor_initializes_correctly) {
+	PIVector2D<int>::Index idx(5, 10);
+	EXPECT_TRUE(idx.isValid());
+	EXPECT_FALSE(idx.isNotValid());
+	EXPECT_EQ(idx.row, 5);
+	EXPECT_EQ(idx.col, 10);
+}
+
+TEST(Vector2DIndexTest, isValid_returns_true_for_non_negative_values) {
+	PIVector2D<int>::Index idx(0, 0);
+	EXPECT_TRUE(idx.isValid());
+	EXPECT_FALSE(idx.isNotValid());
+}
+
+TEST(Vector2DIndexTest, isNotValid_returns_true_for_negative_values) {
+	PIVector2D<int>::Index idx1;
+	EXPECT_TRUE(idx1.isNotValid());
+
+	PIVector2D<int>::Index idx2(-1, 5);
+	EXPECT_TRUE(idx2.isNotValid());
+
+	PIVector2D<int>::Index idx3(5, -1);
+	EXPECT_TRUE(idx3.isNotValid());
+}