fix: complete collision — checks both top row and bottom cell

Added grid[py+2] at px+1 check for the T-piece's protruding
bottom cell. Collision now checks 4 cells total per piece:
- grid[py+1] at px, px+1, px+2 (top row destination)
- grid[py+2] at px+1 (bottom cell destination)

This prevents pieces from passing through the angled/protruding
parts of frozen T-shaped blocks.

examples/game-tetris.ds

@@ -85,14 +85,16 @@ on keydown(ev) -> paused = if ev.key == "p" then (if paused then 0 else 1) else
 -- ================================================================
 
 -- Collision detection: can the piece move down?
--- T-piece at py has top cells at row py, bottom cell at py+1.
+-- T-piece at py: top row at py (3 cells), bottom cell at py+1 (1 cell).
--- To move down: top cells move to py+1, bottom cell to py+2.
+-- Moving down: top row -> py+1, bottom cell -> py+2.
--- Check grid[py+1] at px, px+1, px+2 (top row would land here).
+-- Must check BOTH:
--- Bottom wall: py >= 18 (piece can't go lower).
+--   grid[py+1] at px, px+1, px+2  (top row destination)
+--   grid[py+2] at px+1            (bottom cell destination)
+-- Bottom wall: py >= 18.
 let blocked = 0
 
--- Check collision against the CORRECT row: grid[py+1] for top cells
+-- Full collision: top row (grid[py+1]) + bottom cell (grid[py+2])
-every 33 -> blocked = if py > 17 then 1 else (if py == 17 then (if g18[px] > 0 then 1 else (if g18[px + 1] > 0 then 1 else (if g18[px + 2] > 0 then 1 else 0))) else (if py == 16 then (if g17[px] > 0 then 1 else (if g17[px + 1] > 0 then 1 else (if g17[px + 2] > 0 then 1 else 0))) else (if py == 15 then (if g16[px] > 0 then 1 else (if g16[px + 1] > 0 then 1 else (if g16[px + 2] > 0 then 1 else 0))) else (if py == 14 then (if g15[px] > 0 then 1 else (if g15[px + 1] > 0 then 1 else (if g15[px + 2] > 0 then 1 else 0))) else (if py == 13 then (if g14[px] > 0 then 1 else (if g14[px + 1] > 0 then 1 else (if g14[px + 2] > 0 then 1 else 0))) else (if py == 12 then (if g13[px] > 0 then 1 else (if g13[px + 1] > 0 then 1 else (if g13[px + 2] > 0 then 1 else 0))) else 0))))))
+every 33 -> blocked = if py > 17 then 1 else (if py == 17 then (if g18[px] > 0 then 1 else (if g18[px + 1] > 0 then 1 else (if g18[px + 2] > 0 then 1 else (if g19[px + 1] > 0 then 1 else 0)))) else (if py == 16 then (if g17[px] > 0 then 1 else (if g17[px + 1] > 0 then 1 else (if g17[px + 2] > 0 then 1 else (if g18[px + 1] > 0 then 1 else 0)))) else (if py == 15 then (if g16[px] > 0 then 1 else (if g16[px + 1] > 0 then 1 else (if g16[px + 2] > 0 then 1 else (if g17[px + 1] > 0 then 1 else 0)))) else (if py == 14 then (if g15[px] > 0 then 1 else (if g15[px + 1] > 0 then 1 else (if g15[px + 2] > 0 then 1 else (if g16[px + 1] > 0 then 1 else 0)))) else (if py == 13 then (if g14[px] > 0 then 1 else (if g14[px + 1] > 0 then 1 else (if g14[px + 2] > 0 then 1 else (if g15[px + 1] > 0 then 1 else 0)))) else (if py == 12 then (if g13[px] > 0 then 1 else (if g13[px + 1] > 0 then 1 else (if g13[px + 2] > 0 then 1 else (if g14[px + 1] > 0 then 1 else 0)))) else 0))))))
 
 -- Gravity tick
 every 33 -> gravityTick = if paused then gravityTick else (if gameOver then gravityTick else gravityTick + 1)