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.md

@@ -14,6 +14,7 @@ _Generated by `lib/artdag/conformance.sh`_
 | serialize | 13 | 0 | 13 |
 | stats | 12 | 0 | 12 |
 | fault | 14 | 0 | 14 |
+| post | 12 | 0 | 12 |
 | maude-optimize | 40 | 0 | 40 |
 | schedule | 15 | 0 | 15 |
-| **Total** | **213** | **0** | **213** |
+| **Total** | **225** | **0** | **225** |