Use MSG_WriteDeltaEntity/MSG_ReadDeltaEntity for entity state
serialization. Only changed fields are written per frame. PVS
fields (svFlags, linked, origin, absmin, absmax) also use a
1-bit change flag to skip when unchanged.
Reduces a 10-second demo from ~1400KB to ~52KB (27x smaller).
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>