Function detect 

pub fn detect(
    unit: &FnUnit<'_>,
    imports: &Imports,
    module_bindings: &HashSet<String>,
    span: &SpanIndex<'_>,
) -> Vec<(Effect, bool)>

Detect mutation effects in unit’s own body, with escape analysis.

Returns (Effect, contained) pairs. The bool is the containment flag — true means the write is bounded to this function’s scope (local init or constructor init); false means it escapes.

Task 9 consumes the contained flags to apply boundary-containment discounts.