Fix O(n²) item cramp check and cache per-cycle display names

[wling-art]

• isCramping scanned every item entity against every other item entity in the world each update cycle. Bucket items by block position once per cycle so it only scans the 3x3x3 neighbourhood.
• getDisplayName() was recomputed (NMS -> JSON -> Adventure) for every item entity every cycle, even when many entities shared an identical stack (e.g. mob farm drops). Cache results in a map scoped to a single update cycle, discarded afterwards.