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>

map_manager.lua

@@ -217,12 +217,10 @@ function MapManager:drawRoomLayer(roomData, x, y, size, color, alpha)
 				love.graphics.setColor(r, g, b, alpha)
 				local sprite = loadSprite(props.sprites[color])
 				local dx, dy = x + (object.x - 1) * 16 * scale, y + (object.y - 1) * 16 * scale
-				if props.goopy and drawGoopySprite then
+				if object.adhesion == "gluey" and drawGlueySprite then
+					drawGlueySprite(sprite, props.cells and props.cells[color], dx, dy, scale)
+				elseif object.adhesion == "sticky" and drawGoopySprite then
 					drawGoopySprite(sprite, dx, dy, scale)
-				elseif props.skewer and drawSkewerSprite then
-					drawSkewerSprite(sprite, props.cells and props.cells[color], dx, dy, scale)
-				elseif props.spiky and drawSpikySprite then
-					drawSpikySprite(sprite, props.cells and props.cells[color], dx, dy, scale)
 				else
 					love.graphics.draw(sprite, dx, dy, 0, scale, scale)
 				end