Phase J — Find implementations via the salsa-memoized workspace aggregate.
Uses the pre-built subtypes_of[word] reverse map for O(matches) lookups,
with an additional pass over the FQN’s subtypes_of entry when the caller
supplied one (covers classes that wrote out the fully-qualified form in
their extends/implements clause). Replaces the old
find_implementations_from_index which walked every file’s classes.