Three projections sum independently: xy-plane counts non-zero cells, yz-plane sums per-row max heights, zx-plane sums per-column max heights.

For each cell grid[i][j], accumulate height>0 toward top. Track rowMax[i] = max(rowMax[i], h) and colMax[j] = max(colMax[j], h).

O(n²) single pass fills all three sums. Grid is n x n, n ≤ 50. No edge cases beyond empty grid (all zeros, all projections = 0).