Range for loop for statements

[Alvov1]

Closes #181

Adds a nested RowIterator class to Statement that enables range-based for loops over query results:

SQLite::Statement query(db, "SELECT id, name FROM test");
for (SQLite::Statement& row : query)
{
    std::cout << row.getColumn(0).getInt() << "\n";
}

Notes:

• begin() calls reset() automatically, so re-iterating the same Statement always starts from the beginning
• Bindings set before the loop are preserved across reset()
• Empty result sets are handled correctly (loop body never executes)
• RowIterator satisfies std::input_iterator_tag with full iterator traits