Function glm_vision_enabled_from 

pub fn glm_vision_enabled_from(value: Option<&str>) -> bool

The policy half of glm_vision_enabled, split out so both states are testable — setting an environment variable is unsafe in this edition and a test that did it would race every other test in the binary.

Only an explicit affirmative enables. Anything else — unset, empty, 0, off, a typo — leaves the tower off, because the failure mode of guessing wrong in that direction is an OOM on a certified serve.