Replaces GameWindow._entities flat list with a per-landblock dict
keyed by landblock id. The flat entity view is rebuilt on add/remove
so the renderer keeps its simple "iterate Entities" loop. Also
provides AppendLiveEntity for the CreateObject path that spawns
entities into an already-loaded landblock after hydration.
Not thread-safe — all mutation is render-thread only.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
