[ty] Avoid storing redundant reachability indexes (#25453)

## Summary

When all reachability nodes for a scope are retained, their IDs already
match their positions in the stored node array. In that case, this PR
avoids storing an extra bit vector for mapping node IDs back to the
array. We only allocate that mapping when unused intermediate nodes were
removed.

This reduces retained memory for reachability constraints while
preserving the existing compaction for scopes that need it. It also
moves node lookup into `ReachabilityConstraints::get_interior_node`, so
both paths use the same lookup logic.