artdag: public API facade lib/artdag/api.sx — load list + surface index

Reference index (matching datalog/persist convention): canonical load order and
the full public surface across all 10 modules, plus artdag/version. Wired into the
conformance load list. Total 158/158 unchanged.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lib/artdag/conformance.sh

@@ -51,6 +51,7 @@ run_suite() {
 (load "lib/artdag/serialize.sx")
 (load "lib/artdag/stats.sx")
 (load "lib/artdag/fault.sx")
+(load "lib/artdag/api.sx")
 (epoch 2)
 (eval "(define artdag-test-pass 0)")
 (eval "(define artdag-test-fail 0)")