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/scoreboard.md

@@ -1,6 +1,6 @@
 # datalog scoreboard
 
-**114 / 114 passing** (0 failure(s)).
+**124 / 124 passing** (0 failure(s)).
 
 | Suite | Passed | Total | Status |
 |-------|--------|-------|--------|
@@ -10,3 +10,4 @@
 | eval | 15 | 15 | ok |
 | builtins | 19 | 19 | ok |
 | semi_naive | 8 | 8 | ok |
+| negation | 10 | 10 | ok |