Tailstech Slabbed perf5 experimental build

Placement semantics:
- only real player BlockItem placements can opt into Slabbed
- inheritance occurs only through explicit UP/DOWN face placement onto a persisted player-created Slabbed slab/tower
- side placement does not inherit merely because a slab exists below
- existing blocks are not retroactively pulled by a later slab
- client keeps a short exact-position placement hint to avoid first-frame high/falling mesh flicker before attachment sync
