host P0.1: publish-activity contract for federated composition-flows

Business logic as federated composition-flows (plans/business-logic-fed-flows.md). P0.1: the
host describes a published post as a fed-sx activity — host/blog--publish-activity(slug) →
{:type "create" :actor "site" :id <CID> :object {:type "article" :slug :category}} — the
exact shape next/'s trigger machinery consumes (verified: next/tests/triggers_e2e.sh 10/10).
category (drives the flow branch: newsletter suspends / urgent fires / else skip) comes from
the "category" field-value, else the first tag, else "urgent". + host/blog--post-category.

Design decided: activity log = every CID delta (event source); triggers = declared subscriptions
(DefineTrigger); flows hybrid (SX composition for simple via the execute-fold, named Erlang flows
for complex); federated execution = Erlang (next/); the type carries content+relations+behavior.

blog 200/200 (+3: contract, category fallback, missing-post nil).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

plans/business-logic-fed-flows.md

@@ -0,0 +1,65 @@
+# Business logic as federated composition-flows
+
+**Vision:** business logic is part of an object's composition — declared on its **type**,
+alongside content grammar + allowed relations. State changes federate over fed-sx; federated
+services execute the business logic as durable **flows**.
+
+**Design (decided 2026-07-02):**
+- **Activity log = every CID delta** — the federated event source of truth. Any content change
+  (new CID, per host/blog--cid-of: content in the record → CID; relations are external → not).
+- **Triggers = declared subscriptions** — a type declares named triggers (on-publish, on-relate,
+  …); flows fire only on matching ones. (fed-sx `DefineTrigger`: activity-type → flow-name + guard
+  + actor-scope.) Log complete, execution precise.
+- **Flows = hybrid** — simple logic authored as **SX composition** (the execute-fold: effect/alt/
+  each — already live in /workflow-demo); an escape hatch to named **Erlang flows**
+  (next/flow/*.erl, effect-as-data, deterministic replay, suspend/resume) for the complex.
+- **Federated execution = Erlang** (`next/` fed-sx Milestone-1 kernel: trigger_registry +
+  flow_dispatch + pipeline post-append fan-out). Authoring stays SX; the fed-sx activity is the
+  bridge. flow-on-sx (Scheme, lib/flow) remains for purely-local durable logic.
+- **The type carries its whole contract:** fields+grammar (content) · allowed relations
+  (external) · triggers+flows (behavior). All composition, all editable in the type-def editor.
+
+**Verified baseline:** `next/tests/triggers_e2e.sh` = 10/10 — publish activity → trigger →
+blog_publish_digest flow (urgent/newsletter-suspend/draft-skip/guard-reject/dedup). This is the
+reference P0 wires the live host onto.
+
+## P0 — publish workflow, end-to-end (spike)
+Prove: live host publishes a post → fed-sx activity → on-publish trigger → blog_publish_digest.
+
+- [x] **P0.1 — the publish-activity contract (SX side).** host/blog--publish-activity(slug):
+  a post record → the fed-sx activity {:type "create" :actor "site" :id <CID> :object {:type
+  "article" :category … :slug …}}. category from field-value "category", else first tag, else
+  "urgent". + host/blog--post-category. blog 200/200 (3 tests). DONE 2026-07-02.
+- [ ] **P0.2 — the dispatch bridge.** On publish (host/blog-edit-submit status→published, or a
+  dedicated publish action), emit the activity into the trigger machinery. RUNTIME CHOICE for P0:
+  in-process — serve.sh loads next/ kernel + registers the on-publish trigger; host emits via the
+  erlang-on-sx bridge (erlang-eval-ast pipeline:apply_triggers). (P3 replaces this with real fed-sx
+  delivery over next/kernel/http_server.erl.)
+- [ ] **P0.3 — the effect is visible.** The flow's digest_sent effect surfaces (a durable record /
+  a DigestSent activity back in the log / shown on the post or a /flows page). Close the local loop.
+
+## P1 — types declare behavior (generalize)
+- [ ] A Composition field / the type carries a :triggers list (on-publish → flow-name + guard) —
+  edited in the type-def editor, like grammar + relations.
+- [ ] A fold turns a type's declared behavior into DefineTrigger + flow registrations at boot.
+- [ ] Simple flows authored as SX composition (execute-fold) → compiled/dispatched to the engine;
+  complex ones reference named Erlang flows.
+
+## P2 — state-change → activity emission (the CID-delta event source)
+- [ ] Every content mutation (new CID) appends a state-change activity to the log. Define the
+  activity envelope (verb, actor, object CID, prev CID, delta summary).
+- [ ] Wire the host's write path (put!/set-comp!/edit-submit) to the append.
+
+## P3 — federation proper
+- [ ] Activities cross peers via next/ delivery (http_server / outbox / follower_graph). A remote
+  service's trigger_registry fires the flow. Everything works over fed-sx.
+
+## P4 — close the loop
+- [ ] Flow effects mutate objects back durably (a flow's DescribeEffects → host writes / new
+  activities), so business logic can change state, which federates, which triggers more flows.
+
+## Progress log (newest first)
+- 2026-07-02 — P0.1 done. host/blog--publish-activity + host/blog--post-category; the publish
+  contract in SX, 200/200. Verified next/ triggers e2e baseline 10/10. Roadmap anchored. NEXT:
+  P0.2 the dispatch bridge (in-process: serve.sh loads next/ kernel + registers the on-publish
+  trigger; host emits the activity via the erlang-on-sx bridge to pipeline:apply_triggers).