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>

plans/artdag-on-sx.md

@@ -31,7 +31,7 @@ edges.
 
 ## Status (rolling)
 
-`bash lib/artdag/conformance.sh` → **213/213** (12 suites: dag, analyze, plan, execute, optimize, fed, cost, serialize, stats, fault, maude-optimize, schedule)
+`bash lib/artdag/conformance.sh` → **225/225** (13 suites: dag, analyze, plan, execute, optimize, fed, cost, serialize, stats, fault, post, maude-optimize, schedule)
 
 Base roadmap (Phases 1–6) COMPLETE + Phase 7 (maude rule-based optimization) COMPLETE
 (only optional miniKanren scheduling remains). Now hardening only.
@@ -117,10 +117,14 @@ loop's `GET/POST /feed`. The engine already has the primitives; the wrapping liv
 - **federation already mirrors the feed** — `fed-export`/`fed-import` are trust-gated with
   provenance; a re-posted job dedupes/cache-hits by global content-id.
 
-Boundary: host loop owns the `dag ⇄ feed-object` adapter; `lib/artdag` stays the engine.
-Candidate artdag-side affordance (only if wanted here): a thin `job->post-object` /
-`post-object->job` projection so host never reaches into wire internals. Not yet built —
-flagged, not scheduled.
+Boundary: host loop owns the activity wrapping (actor/verb/at/tags); `lib/artdag` owns the
+job⇄object projection. **BUILT — `lib/artdag/post.sx`** (post suite 12/12): a post object is
+`{:type "artdag/job" :id <output content-id> :wire <dag->wire>}` — `job->post-object`,
+`post-object->job`, `post-object-verify` (wire ids intact + the post id is produced by the
+job; rejects tampered params and bogus ids), `job->post-string`/`post-string->object` for the
+feed/SXTP body, and `post-run` (a peer decodes → runs → result at the post id, content-address
+cache-hitting what it already has). The host loop drops the object into a feed activity's
+`:object`; post-id = content-id = the AP object id.
 
 ## Phase 1 — DAG model + content addressing
 
@@ -237,6 +241,19 @@ be an op token.
 
 ## Progress log
 
+- **2026-06-28 Forward — job as a feed post object** (post suite 12/12, total 225/225).
+  `lib/artdag/post.sx`: the artdag-side projection for "a job is a type of post" (per the
+  host loop). `job->post-object dag output-name` → `{:type "artdag/job" :id <content-id of
+  output> :wire <dag->wire>}` — the post/object id IS the output node's content-id (= the
+  natural AP object id), the body is serialize.sx's self-describing wire. `post-object-verify`
+  binds the claimed id to the payload (every record's id recomputes + the post id is present
+  among them) — rejects a param tampered under a stale id and a bogus post id.
+  `job->post-string`/`post-string->object` carry it in a feed activity / SXTP body;
+  `post-object->job` decodes; `post-run post runner cache` lets a peer decode → run → read the
+  result at the post id (content-addressed, so a warm peer cache-hits what it already holds).
+  The activity wrapper (actor/verb/at/tags) stays on the host/feed side — this is only
+  job⇄object. Reuses serialize.sx + execute.sx; no new substrate.
+
 - **2026-06-28 Phase 3/7 — miniKanren CLP(FD) scheduler** (schedule suite 15/15, total
   213/213). `lib/artdag/schedule.sx` on `lib/minikanren` (read-only substrate): each node
   gets a slot var in `[1..max-slots]`, every edge `(input->node)` imposes `fd-lt