ImmediatelyFast Forge 1.16.5 Zythera-safe subset

Based on low-risk behavior from ImmediatelyFast 1.1.12 (LGPL-3.0).
Original project: https://github.com/RaphiMC/ImmediatelyFast

Included in this 1.16.5 adaptation:
- TextureManager released-texture map cleanup.
- Optional 2048 font atlas (disabled by default).

Intentionally NOT backported in this build:
- fast_buffer_upload
- native BufferBuilder pooling
- HUD batching
- map-atlas generation
- GL error-check changes
- other 1.18 renderer-specific mixins

Those features touch APIs heavily rewritten between 1.16.5 and 1.18.2 and overlap
with Embeddium/Oculus in ZytheraCraft. Excluding them is deliberate to prioritize
startup/rendering safety over pretending this is a full byte-for-byte port.
