#ensureDir checked #initialized then set it across an await
#scanExistingIds() gap, so two concurrent first-use save/allocatePath
callers both re-seeded #nextId=maxId+1 and allocateId() handed both the
same id — silently overwriting the first artifact (same toolType) or
making artifact:// resolution ambiguous (different toolTypes).
Memoize the initial scan as a single in-flight #initPromise so all
concurrent callers share one initialization and receive distinct ids.
Fixes#4091
roboomp