```python
def gen_plan(state: BridgeState):
    
    def calculate_centroid(blocks):
        """Calculate the centroid of the blocks."""
        x_coords = [block.x_pos for block in blocks]
        y_coords = [block.y_pos for block in blocks]
        centroid_x = sum(x_coords) / len(blocks)
        centroid_y = sum(y_coords) / len(blocks)
        return centroid_x, centroid_y

    def sort_blocks_by_distance_to_centroid(blocks, centroid):
        """Sort blocks by their distance to the centroid."""
        blocks.sort(key=lambda block: ((block.x_pos - centroid[0]) ** 2 + (block.y_pos - centroid[1]) ** 2) ** 0.5)
        return blocks

    def generate_line_actions(sorted_blocks, centroid):
        """Generate actions to align blocks in a straight line."""
        actions = []
        # Determine the direction of the line
        direction_vector = [1, 0]  # Assuming a horizontal line for simplicity
        start_point = [centroid[0] - direction_vector[0] * len(sorted_blocks) * 0.08 / 2,
                       centroid[1] - direction_vector[1] * len(sorted_blocks) * 0.08 / 2]
        
        for i, block in enumerate(sorted_blocks):
            target_x = start_point[0] + i * 0.08 * direction_vector[0]
            target_y = start_point[1] + i * 0.08 * direction_vector[1]
            actions.append(Action("push_motion", [block.x_pos, block.y_pos, target_x, target_y]))
        
        return actions

    blocks = state.frames[:-1]  # Exclude the last frame which is not a block
    centroid = calculate_centroid(blocks)
    sorted_blocks = sort_blocks_by_distance_to_centroid(blocks, centroid)
    actions = generate_line_actions(sorted_blocks, centroid)
    
    return actions
```