fix: collision off-by-one — pieces now stack adjacently

Collision was checking grid[py+2] instead of grid[py+1].
Pieces stopped one row too early, creating gaps.
Now checks grid[py+1] (where top cells would land) with full
3-column width (px, px+1, px+2).
Pieces now stack directly on top of each other.

examples/game-tetris.ds

@@ -84,20 +84,15 @@ on keydown(ev) -> paused = if ev.key == "p" then (if paused then 0 else 1) else
 -- LAYER 4: DERIVED SIGNALS — Collision, gravity, lock, line clear
 -- ================================================================
 
--- Collision detection signal: is the row BELOW the piece occupied?
--- For a standard piece (3 wide, 2 tall like T), the bottom cells are at py+1.
--- We check if grid row py+2 has any blocks at the piece's x positions.
--- `blocked` = 1 means can't move down, 0 means safe to drop.
---
--- Since grid rows are individual signals, we branch on py to check the right row.
--- Also blocked at the bottom wall (py >= 18 for standard pieces).
+-- Collision detection: can the piece move down?
+-- T-piece at py has top cells at row py, bottom cell at py+1.
+-- To move down: top cells move to py+1, bottom cell to py+2.
+-- Check grid[py+1] at px, px+1, px+2 (top row would land here).
+-- Bottom wall: py >= 18 (piece can't go lower).
 let blocked = 0
 
--- Check collision: at bottom wall OR frozen cells below
--- T/J/L/S/Z pieces: bottom at py+1, so check py+2
--- I piece: bottom at py+1, check py+2
--- Piece bottom row = py+1, next row down = py+2
-every 33 -> blocked = if py > 17 then 1 else (if py == 17 then (if g19[px] > 0 then 1 else (if g19[px + 1] > 0 then 1 else 0)) else (if py == 16 then (if g18[px] > 0 then 1 else (if g18[px + 1] > 0 then 1 else 0)) else (if py == 15 then (if g17[px] > 0 then 1 else (if g17[px + 1] > 0 then 1 else 0)) else (if py == 14 then (if g16[px] > 0 then 1 else (if g16[px + 1] > 0 then 1 else 0)) else (if py == 13 then (if g15[px] > 0 then 1 else (if g15[px + 1] > 0 then 1 else 0)) else (if py == 12 then (if g14[px] > 0 then 1 else (if g14[px + 1] > 0 then 1 else 0)) else 0))))))
+-- Check collision against the CORRECT row: grid[py+1] for top cells
+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))))))
 
 -- Gravity tick
 every 33 -> gravityTick = if paused then gravityTick else (if gameOver then gravityTick else gravityTick + 1)