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/scoreboard.md

@@ -1,6 +1,6 @@
 # datalog scoreboard
 
-**134 / 134 passing** (0 failure(s)).
+**143 / 143 passing** (0 failure(s)).
 
 | Suite | Passed | Total | Status |
 |-------|--------|-------|--------|
@@ -12,3 +12,4 @@
 | semi_naive | 8 | 8 | ok |
 | negation | 10 | 10 | ok |
 | aggregates | 10 | 10 | ok |
+| api | 9 | 9 | ok |