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/object_attributes.lua

@@ -1,7 +1,5 @@
 require "libs/TSerial"
 
-local BoxVariants = require "box_variants"
-
 local targetFolder = "art"
 local reanalyzeCells = true
 
@@ -77,5 +75,5 @@ function getAllAssets()
 	synthetic("wired_switch", "switch", "wired_switch")
 	synthetic("wired_door", "door", "wired_door")
 
-	return BoxVariants.addTo(globalAssetProperties)
+	return globalAssetProperties
 end