Epic 2 — Range & performance: scan far, never lag

[JROtto5]

Goal: cover more ground without frame drops.

• Move the scan onto a background thread (no main-thread stalls)
• Incremental scan: only re-check chunks that changed/loaded, cache the rest
• Spatial index (per-chunk block-entity lists) instead of full rescans
• Scan cadence adapts to movement speed
• Configurable scan radius decoupled from render distance
• Skip empty chunks fast
• Debounce so standing still costs ~0
• Profile + budget: cap ms/tick spent scanning
• Optional deep-scan burst on a keypress
• Memory cap on the container index