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>

movement.lua

@@ -1,6 +1,7 @@
 -- Finds the complete set of blocks that must move together for one grid step.
 -- A block joins the set when it is in another block's destination (ordinary
--- pushing), is softly grabbed by sticky/spiky, or touches a rigid skewer.
+-- pushing), is softly grabbed by a sticky block, or is rigidly welded to a
+-- gluey block.
 local Movement = {}
 
 local adjacentDirections = {
@@ -18,15 +19,11 @@ local function isPassable(entity)
 	return entity.canPassOver and entity:canPassOver()
 end
 
-local function isSkewer(entity)
-	return entity.isSkewer and entity:isSkewer()
+local function isGluey(entity)
+	return entity.isGluey and entity:isGluey()
 end
 
-local function isSpiky(entity)
-	return entity.isSpiky and entity:isSpiky()
-end
-
--- Build one rigid component: ordinary pushes are required, and skewers join
+-- Build one rigid component: ordinary pushes are required, and gluey blocks weld
 -- along every edge. `alreadyMoving` contains cells that will be vacated during
 -- this same step, so a later pulled component may move into them.
 local function findRigidGroup(startCells, direction, entityAt, isInBounds, alreadyMoving)
@@ -59,7 +56,7 @@ local function findRigidGroup(startCells, direction, entityAt, isInBounds, alrea
 				table.insert(pendingCells, projectedCell)
 			end
 
-			-- Skewers make every edge contact part of this rigid component.
+			-- Gluey blocks make every edge contact part of this rigid component.
 			for _, occupiedCell in ipairs(entity:getOccupiedCells()) do
 				for _, adjacentDirection in ipairs(adjacentDirections) do
 					local adjacentCell = {
@@ -69,7 +66,7 @@ local function findRigidGroup(startCells, direction, entityAt, isInBounds, alrea
 					if isInBounds(adjacentCell) then
 						local adjacentEntity = entityAt(adjacentCell)
 						local attached = adjacentEntity and (
-							isSkewer(entity) or isSkewer(adjacentEntity)
+							isGluey(entity) or isGluey(adjacentEntity)
 						)
 						if attached and adjacentEntity ~= entity and adjacentEntity:canMove() and
 							not (alreadyMoving and alreadyMoving[adjacentEntity]) then
@@ -99,9 +96,9 @@ function Movement.findMovableGroup(startCells, direction, entityAt, isInBounds)
 		local entity = movingQueue[nextMover]
 		nextMover = nextMover + 1
 
-		-- Sticky contact is soft only on the trailing face. Spiky contact is soft
-		-- on every face. Either kind simply releases a blocked rigid component,
-		-- without cancelling this entity or another independently pulled branch.
+		-- Sticky contact is soft on every face: it tries to drag each neighbour
+		-- along, but simply releases a blocked rigid component without cancelling
+		-- this entity or another independently pulled branch.
 		for _, occupiedCell in ipairs(entity:getOccupiedCells()) do
 			for _, adjacentDirection in ipairs(adjacentDirections) do
 				local adjacentCell = {
@@ -110,11 +107,8 @@ function Movement.findMovableGroup(startCells, direction, entityAt, isInBounds)
 				}
 				if isInBounds(adjacentCell) then
 					local adjacentEntity = entityAt(adjacentCell)
-					local trailing = adjacentDirection.x == -direction.x and
-						adjacentDirection.y == -direction.y
 					local softContact = adjacentEntity and (
-						isSpiky(entity) or isSpiky(adjacentEntity) or
-						(trailing and (isSticky(entity) or isSticky(adjacentEntity)))
+						isSticky(entity) or isSticky(adjacentEntity)
 					)
 					if softContact and adjacentEntity ~= entity and
 						not entities[adjacentEntity] and not attemptedPulls[adjacentEntity] and