Chunk streaming concurrency, entity slot fix, and map-data-driven spawns

- Allow 2 chunks to be mid-load concurrently instead of 1 (MAP_CHUNK_LOAD_CONCURRENCY).
- Fix entitySetChunk silently losing track of an entity when its target chunk's
  entity slots are full - it now stays detached (and retries later) instead of
  claiming a chunk that never actually registered it.
- DCF format bumped to v5: chunks can now declare entity spawns (global/NPC via
  the existing entityglobal registry, or one-shot item pickups) and map area
  triggers, resolved via a new callback-ID registry (mapareagloballist.h)
  mirroring the entity one. rpg.c's hardcoded TEST entity/item/area spawns are
  gone - chunk_0_0_0.json now carries that data instead. The player is still
  bootstrapped in code since it isn't map-authored content.

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