Function interpolate_with_lookup 

pub fn interpolate_with_lookup<F>(
    text: &str,
    lookup: F,
) -> Result<String, String>
where
    F: Fn(&str) -> Option<String>,

Iterative leaf-placeholder substitution with escape handling, round cap, and final unresolved-name check. The lookup closure decides where each leaf’s value comes from.

Public so callers like the synthesis-time clause probe can compose their own lookup (parent kernel + workload params + clause probes) without reimplementing the iterative loop.