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/scoreboard.json

@@ -10,10 +10,11 @@
     "serialize": {"pass": 13, "fail": 0},
     "stats": {"pass": 12, "fail": 0},
     "fault": {"pass": 14, "fail": 0},
+    "post": {"pass": 12, "fail": 0},
     "maude-optimize": {"pass": 40, "fail": 0},
     "schedule": {"pass": 15, "fail": 0}
   },
-  "total_pass": 213,
+  "total_pass": 225,
   "total_fail": 0,
-  "total": 213
+  "total": 225
 }