Lazily allocate ChunkEntitySlices state

[wphillipp]

Summary

Lazily allocate ChunkEntitySlices state that is unused for most chunks.

This avoids retaining entity lists, section collections, and lookup maps for empty chunks.

Changes

• Lazily allocate entity lists and section collections.
• Only allocate hard-collision state when needed.
• Allocate type/class maps after the first entity is added.
• Keep individual class lookup collections lazy.
• Keep allocated state after first use to avoid allocation churn.

Memory impact

In a 625 full chunk test, this saved about 0.7 MiB of directly attributable retained memory, or roughly 1.2 KiB per full chunk.

Object	Baseline	Optimized
EntityCollectionBySection	1,440	27
EntityList	628	10
Reference2ObjectOpenHashMap	7,213	5,977
Int2IntOpenHashMap	628	10

Direct heap graph analysis puts the retained saving at about 0.714 MiB per 625 full chunks.

With view-distance=10 and simulation-distance=10, an isolated player keeps about 625 full chunks loaded in this workload. For players far enough apart that their chunk areas do not overlap, this scales roughly to:

Players	Full chunks	Estimated saving
10	6,250	~7.1 MiB
20	12,500	~14.3 MiB
50	31,250	~35.7 MiB
100	62,500	~71.4 MiB

Actual savings will be lower when players are close enough for their loaded chunk areas to overlap.

Validation

Tested entity add/remove, section movement, cross-chunk movement, hard-colliding entities, save/unload, reuse after becoming empty, and unload/reload.

Mixin audit and full build pass.