Function find_implementations_from_workspace 

pub fn find_implementations_from_workspace(
    word: &str,
    fqn: Option<&str>,
    wi: &WorkspaceIndexData,
) -> Vec<Location>

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.