docs(arch): ADR-0008 read projection store + demo note for the event path (refs #7)

ADR-0008 records the read-projection design: one rebuildable store shared by the Event
Subscriber (writer) and projection-api (reader) as one CQRS bounded context (reconciled
with §8.5), idempotency + rebuild from the notification log (no OpenZaak access, §8.1),
the deferred bsn/naam, and the new EF Core + Npgsql dependency. Add a demo-script entry
walking the OZ→NRC→subscriber→projection-api path and wire both into the MkDocs nav.

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

docs/demo-script.md

@@ -0,0 +1,35 @@
+# Demo script
+
+A running log of demoable outcomes, one section per slice. Each entry is a short,
+copy-pasteable walkthrough against a local `make up` stack.
+
+---
+
+## S-06 — Event Subscriber + read projection
+
+**Outcome:** a zaak created in OpenZaak flows through NRC to the Event Subscriber, which
+projects it into a rebuildable read projection the projection-api serves.
+
+**The path:** OpenZaak → (notification) NRC → (abonnement callback) Event Subscriber →
+`register_projection` → projection-api `GET /register`.
+
+```bash
+# 1. Bring the full stack up (seeds config, builds our services, waits for health).
+make up
+
+# 2. Register the Event Subscriber's abonnement and create a zaak, then read it back.
+#    (The verify-projection check does exactly this end-to-end and asserts the result.)
+make verify-projection            # → "OK — projection-api serves zaak <uuid> with status INGEDIEND"
+
+# 3. Observe the projection directly via the read API (host port 8120).
+curl -fsS http://localhost:8120/register | jq
+# → [ { "id": "<zaak-uuid>", "status": "INGEDIEND", "bsn": null, "naamPlaceholder": null } ]
+
+# 4. Idempotency + rebuild: replays don't duplicate; a rebuild repopulates from the
+#    notification log (no OpenZaak access needed — ADR-0008).
+curl -fsS -X POST http://localhost:8110/admin/rebuild   # Event Subscriber, host port 8110
+curl -fsS http://localhost:8120/register | jq 'length'  # → unchanged
+```
+
+> `bsn` / `naam_placeholder` are deferred (ADR-0008) — the notification doesn't carry them and
+> the subscriber may not read OpenZaak directly (§8.1). They surface in a later slice.