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.