datalog: stratified negation (Phase 7, 124/124)

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.

lib/datalog/conformance.conf

@@ -9,6 +9,7 @@ PRELOADS=(
   lib/datalog/unify.sx
   lib/datalog/db.sx
   lib/datalog/builtins.sx
+  lib/datalog/strata.sx
   lib/datalog/eval.sx
 )
 
@@ -19,4 +20,5 @@ SUITES=(
   "eval:lib/datalog/tests/eval.sx:(dl-eval-tests-run!)"
   "builtins:lib/datalog/tests/builtins.sx:(dl-builtins-tests-run!)"
   "semi_naive:lib/datalog/tests/semi_naive.sx:(dl-semi-naive-tests-run!)"
+  "negation:lib/datalog/tests/negation.sx:(dl-negation-tests-run!)"
 )