Replace box variants with a moveable adhesion property

Collapse the spiky/sticky/skewer box-variant assets into a single per-
instance `adhesion` property on any moveable block, chosen in the editor
via a dropdown next to the colour picker and stored in the .sav alongside
the class.

- spiky -> sticky: soft grab on every true edge (goopy shader)
- skewer -> gluey: rigid weld on every true edge (spiked outline)
- old trailing-face sticky is removed; its saves fold into new sticky

Move the spike geometry + drawGlueySprite into shaders.lua, fold drawing
and the adhesion flag into GenericMoveable, and delete the three
Generic{Sticky,Spiky,Skewer} classes and box_variants.lua. Room:createEntity
normalises legacy spiky_/skewer_/sticky_ class prefixes so old saves load,
and serialization emits `adhesion`. World/map previews and the movement
proxy follow the rename.

Backfill rooms/*.sav to the new format via tools/backfill_adhesion.lua
(41 objects across 8 files). Update tests to the two-mode model; the two
tests asserting the removed lateral-shear limitation are dropped and a
legacy-load migration test added. 27/27 pass.

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

generic_moveable.lua

@@ -2,7 +2,7 @@ require "entity"
 
 GenericMoveable = class("GenericMoveable", Entity)
 
-function GenericMoveable:initialize(x, y, color, sprite, name, cellShape)
+function GenericMoveable:initialize(x, y, color, sprite, name, cellShape, adhesion)
 	Entity.initialize(self, {
 		name = name,
 		x = x,
@@ -10,7 +10,8 @@ function GenericMoveable:initialize(x, y, color, sprite, name, cellShape)
 		color = color,
 		collision = "moveable",
 		sprite = sprite,
-		cellShape = cellShape
+		cellShape = cellShape,
+		adhesion = adhesion
 	})
 end
 
@@ -18,6 +19,25 @@ function GenericMoveable:update()
 
 end
 
+-- Adhesion drives the look as well as the mechanic: gluey blocks get spiked
+-- teeth on their true contour, sticky blocks get the wobbling goopy shader, and
+-- plain moveables draw normally.
+function GenericMoveable:draw(channel)
+	local sprite = self.sprites[channel]
+	if not sprite then return end
+	love.graphics.setColor(gColor[channel]:set())
+	local p = self:getDrawPos()
+	local x = p.x + self.drawOffset.x * drawScale
+	local y = p.y + self.drawOffset.y * drawScale
+	if self.adhesion == "gluey" and drawGlueySprite then
+		drawGlueySprite(sprite, self:getCellShape(), x, y, drawScale)
+	elseif self.adhesion == "sticky" and drawGoopySprite then
+		drawGoopySprite(sprite, x, y, drawScale)
+	else
+		love.graphics.draw(sprite, x, y, 0, drawScale, drawScale)
+	end
+end
+
 function GenericMoveable:tick()
 
-end
+end