Workspace-index-backed class lookup: maps a short class name to the
ParsedDoc that defines it. Used by all_instance_members and
all_static_members to avoid scanning all workspace docs linearly.
Workspace-index-backed class name search: given a typed prefix, returns
(short_name, kind, fqn) for every workspace class whose short name
starts with it — including classes defined in files that are not open
in the editor (e.g. vendor/). Complements other_docs, which only
covers currently open documents.