New lib/datalog/strata.sx: dl-build-dep-graph (relation -> deps with
:neg flag), Floyd-Warshall reachability, SCC-via-mutual-reach for
non-stratifiability detection, iterative dl-compute-strata, and
dl-group-rules-by-stratum.
eval.sx refactor:
- dl-saturate-rules! db rules — semi-naive worker over a rule subset
- dl-saturate! db — stratified driver. Rejects non-stratifiable
programs at saturation time, then iterates strata in order
- dl-match-negation — succeeds iff inner positive match is empty
Order-aware safety in dl-rule-check-safety (Phase 4) already
required negation vars to be bound by a prior positive literal.
Stratum dict keys are strings (SX dicts don't accept ints).
Phase 6 magic sets deferred — opt-in path, semi-naive default
suffices for current workloads.
