The empty-tree grammar relaxation (create-modal episodes seed) must not
leak into the search deep-link contract: qf={"mode":"all","children":[]}
as a QUERY would silently mean "match everything". The seed's meaning —
a rules-less collection matching nothing — lives in the stored tree and
its composers; the query entry point now rejects a rule-less tree
explicitly, preserving the pinned 400 that TestFilterQFValidationAndPrecedence
guards (the test caught the leak during this round's full-suite run).
John O'Keefe
744d33e13f