Add live Visuals controls (Glow, Trails, Brightness, Size)

A new Visuals panel drives the look in real time so dragging a slider is
instantly visible: Glow (bloom strength) and Brightness (exposure) feed a
new group-3 post-params uniform shared by the fade and composite passes,
Trails sets the HDR scene's per-frame persistence, and Size scales every
creature via the sim uniform. Defaults match the cinematic look, pushed
once at startup.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>