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>

level_editor/gui.lua

@@ -9,10 +9,6 @@ function GUI:initialize(t)
 	self.offset = {x = 0, y = 0}
 	self.name = t.name
 	self.text = t.text
-	self.goopy = t.goopy or false
-	self.skewer = t.skewer or false
-	self.spiky = t.spiky or false
-	self.cells = t.cells
 
 	if not t.onClic then
 		t.onClic = {}
@@ -103,15 +99,7 @@ function GUI:draw()
 	if self.sprites then
 		for color, sprite in pairs(self.sprites) do
 			love.graphics.setColor(gColor[color]:set())
-			if self.goopy and drawGoopySprite then
-				drawGoopySprite(sprite, box.x, box.y, self.scale)
-			elseif self.skewer and drawSkewerSprite then
-				drawSkewerSprite(sprite, self.cells and self.cells[color], box.x, box.y, self.scale)
-			elseif self.spiky and drawSpikySprite then
-				drawSpikySprite(sprite, self.cells and self.cells[color], box.x, box.y, self.scale)
-			else
-				love.graphics.draw(sprite, box.x, box.y, 0, self.scale, self.scale)
-			end
+			love.graphics.draw(sprite, box.x, box.y, 0, self.scale, self.scale)
 		end
 	end