artdag: job-as-post-object projection (post.sx) + 12 tests

lib/artdag/post.sx — the artdag-side projection for 'a job is a type of post' (per the
host loop). job->post-object: {:type artdag/job :id <output content-id> :wire <dag->wire>},
post-id = content-id = natural AP object id. post-object-verify binds the id to the payload
(record ids recompute + post id present), rejecting tampered params/bogus ids. String
transport for the feed/SXTP body; post-run lets a peer decode->run->result, content-address
cache-hitting. Activity wrapping stays host-side. post 12/12, total 225/225.

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

lib/artdag/conformance.sh

@@ -13,7 +13,7 @@ if [ ! -x "$SX_SERVER" ]; then
   exit 1
 fi
 
-SUITES=(dag analyze plan execute optimize fed cost serialize stats fault maude-optimize schedule)
+SUITES=(dag analyze plan execute optimize fed cost serialize stats fault post maude-optimize schedule)
 
 OUT_JSON="lib/artdag/scoreboard.json"
 OUT_MD="lib/artdag/scoreboard.md"
@@ -96,6 +96,7 @@ ${MK_LOADS}
 (load "lib/artdag/serialize.sx")
 (load "lib/artdag/stats.sx")
 (load "lib/artdag/fault.sx")
+(load "lib/artdag/post.sx")
 (load "lib/artdag/api.sx")
 ${BRIDGE_LOAD}
 ${SCHED_LOAD}