Add multi-cell box shapes with per-cell collision

Generate 1x2/2x1 dominoes, 2x2 (square + 4 L-trominoes) and all
connected 3x2/2x3 polyominoes as R/G/B channel masks, reusing the
distance-based concentric-ring format that reproduces box_r/g/b
exactly. Outer silhouette keeps a 1px margin; connected cells stay
seamless. Each shape ships a {class="moveable"} .meta so getAllAssets
auto-registers it as a GenericMoveable.

Wire the occupied-cell shape (precomputed once by getAllAssets via
getCellsFromSprite) through to entities: createEntity now passes the
cached cells as cellShape, so an L/U/notched block collides on its real
cells and its gaps read as empty space. Fix the per-colour cache that
was reset every channel so only one colour survived.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

generic_unmoveable.lua

@@ -2,13 +2,14 @@ require "entity"
 
 GenericUnmoveable = class("GenericUnmoveable", Entity)
 
-function GenericUnmoveable:initialize(x, y, color, sprite)
+function GenericUnmoveable:initialize(x, y, color, sprite, cellShape)
 	Entity.initialize(self, {
 		x = x,
 		y = y,
 		color = color,
 		collision = "unmoveable",
-		sprite = sprite
+		sprite = sprite,
+		cellShape = cellShape
 	})
 end