docs(domain): demo note for submitting a registration (S-05) (refs #6)

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

docs/demo-script.md

@@ -5,6 +5,38 @@ copy-pasteable walkthrough against a local `make up` stack.
 
 ---
 
+## S-05 — BIG Domain Service: submit a registration
+
+**Outcome:** submitting a registration starts a Flowable process; the external-task worker
+opens a zaak via the ACL and records it on the aggregate — the upstream half of the skeleton
+that produces the zaak S-06 then projects.
+
+**The path:** domain `POST /registrations` → Flowable `registratie` process → `OpenZaakAanmaken`
+worker → ACL → OpenZaak; `GET /registrations/{id}` shows the opened zaak (ADR-0009).
+
+```bash
+# 1. Bring the full stack up (seeds config, builds our services, waits for health).
+make up
+
+# 2. Drive the full path end-to-end. This also seeds a published BIG zaaktype and points the
+#    ACL at it (the zaak's zaaktype URL is server-assigned, so it isn't known at bring-up).
+make verify-domain      # → "OK — the domain opened a zaak and recorded it on the registration"
+
+# 3. Submit one yourself (domain on host port 8130). Returns 202 + a Location to read back.
+loc=$(curl -fsS -D - -o /dev/null -X POST http://localhost:8130/registrations \
+  -H 'Content-Type: application/json' -d '{"bsn":"123456782"}' | sed -n 's/\r$//; s/^[Ll]ocation: //p')
+
+# 4. The worker opens the zaak off the request path (eventual consistency, ADR-0009); poll
+#    until zaakUrl is filled. (Step 2 must have run first, so the ACL knows the zaaktype.)
+curl -fsS "http://localhost:8130$loc" | jq
+# → { "registrationId": "...", "status": "Ingediend", "zaakUrl": "http://.../zaken/api/v1/zaken/<uuid>" }
+```
+
+> Registration state is in-memory for this slice (ADR-0009); the rebuildable read model is the
+> projection (S-06), fed by the very zaak this flow opens.
+
+---
+
 ## S-06 — Event Subscriber + read projection
 
 **Outcome:** a zaak created in OpenZaak flows through NRC to the Event Subscriber, which