prop_nxtask_store_test.nx

prop_nxtask_store_test.nx -- property test for nxtask serializer. Property: for any sequence of nxtask operations (add, claim, close, add_dep), serialize -> deserialize round-trip preserves every observable field. If this ever fails, a persisted agent state would silently corrupt on reload. Runs 500 iterations. Each builds a random graph (3-8 tasks, random mutations), serializes, deserializes, compares.