Mutual recursion inlining.

Given a let rec group, this pass identifies helper bindings — those not used by the body (non-roots), not self-recursive, and safe to inline according to the same size/cost heuristics as the main inliner — and inlines them into their callers. This works across independent subgroups within the same let rec, e.g. {even, odd, f, g} where {even, odd} and {f, g} are separate cycles.

No beta reduction is performed here; the resulting applications are left for downstream passes to clean up.