Function render_ground 

pub fn render_ground(template: &str, state: &State) -> String

Render a grounding template against state.

• {key} interpolates the value at key (strings bare, other JSON compact); an absent key renders empty.
• {key?yes:no} renders yes when key is truthy (present and not false/null/0/""), else no.

This is the realization of Effect::ground: a deterministic projection of known State into a steering line, so the model restates facts rather than inventing them.