gull group fixes & improvements
- stronger obstacle avoidance when taking off from land point
- removed per-frame raycasts from the scattering run (might have been the stutter)
- add step height 0.1m for walking gulls, so they stay on the surface they landed on
- landing groups only startle against players within 6m, instead of any target seen in the last 15s
- gull corpses inherit velocity
- try fix "entity_distance" when shooting flying and landing gulls by adjusting bounds depending on state
- groups spawn in deep sea with a hook into DeepSeaManager, since populations make it not naturally spawn
- birds turning into animators are capped per frame because its the heaviest cost for reactivation