📘Lock the piece and clear lines

When a piece lands, merge(grid, piece) writes its 1s into the grid — it's now part of the landscape.

clearLines(grid) scans bottom-up: every full row is removed (splice), an empty row is added on top (unshift), and cleared rows are counted.

grid.splice(r, 1);      // remove the full row
grid.unshift(emptyRow); // empty row on top
cleared = cleared + 1;