datalog: SX-data embedding API (Phase 9, 143/143)

New lib/datalog/api.sx: dl-program-data facts rules takes SX data
lists. Rules accept either dict form or list form using <- as the
rule arrow (since SX parses :- as a keyword). dl-rule constructor
for the dict shape. dl-assert! adds a fact and re-saturates;
dl-retract! drops EDB matches, wipes all rule-headed IDB
relations, and re-saturates from scratch — simplest correct
semantics until provenance tracking arrives.

9 API tests cover ancestor closure via data, dict-rule form,
dl-rule constructor, incremental assert/retract, cyclic-graph
reach, assert into empty, fact-style rule (no arrow), dict
passthrough.

lib/datalog/conformance.conf

@@ -12,6 +12,7 @@ PRELOADS=(
   lib/datalog/aggregates.sx
   lib/datalog/strata.sx
   lib/datalog/eval.sx
+  lib/datalog/api.sx
 )
 
 SUITES=(
@@ -23,4 +24,5 @@ SUITES=(
   "semi_naive:lib/datalog/tests/semi_naive.sx:(dl-semi-naive-tests-run!)"
   "negation:lib/datalog/tests/negation.sx:(dl-negation-tests-run!)"
   "aggregates:lib/datalog/tests/aggregates.sx:(dl-aggregates-tests-run!)"
+  "api:lib/datalog/tests/api.sx:(dl-api-tests-run!)"
 )