docs(workflow): ADR-0016 + demo note for diploma-eligibility DMN (S-13, refs #14)

Record the DMN-as-businessRuleTask decision (from proposal #100) and add the S-13
demo walkthrough showing a foreign diploma routing through CBGV-advies.

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

docs/architecture/adr-0016-diploma-eligibility-dmn.md

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+# ADR-0016: Diploma eligibility is a DMN evaluated inline as a BPMN businessRuleTask
+
+- **Status:** Accepted
+- **Date:** 2026-07-17
+- **Deciders:** Respellion engineering
+- **Relates to:** S-13 (#14); proposal #100. Builds on ADR-0009 (external-task worker / Workflow
+  Client), ADR-0014/0015 (the boundary-event and routing constructs on the registratie process).
+
+## Context
+
+S-13 adds flow 4: a foreign diploma must get an extra CBGV-advies assessment before beoordeling
+(PRD §5). The eligibility decision — domestic goes straight to beoordeling, foreign routes through
+CBGV-advies — needs a home. The Flowable REST app bundles a DMN engine, and the same
+`repository/deployments` machinery that deploys `registratie.bpmn` can deploy a `.dmn`. §8.2 makes
+the Workflow Client the only code that talks to Flowable; the PRD frames the workflow as "BPMN + DMN
+governing the registration workflow" (Flowable as a peer orchestration module).
+
+The issue's wording ("a DMN decision table evaluated by the Domain Service via Workflow Client")
+suggests the domain reaches into Flowable's DMN API to evaluate the decision and feeds the result
+back. That is one option; it is not the only one, and it is not the cleanest.
+
+## Decision
+
+**The diploma-eligibility DMN is deployed to Flowable and evaluated inline by the registratie process
+as a `businessRuleTask`; an exclusive gateway routes on its output. The domain's only new job is to
+carry the diploma origin and pass it into the process as a start variable.**
+
+- **The decision lives in the workflow.** `workflows/diploma-eligibility.dmn` maps `diplomaOrigin`
+  → `route` (`Buitenlands` ⇒ `CBGV_ADVIES`, otherwise `DIRECT`). A `businessRuleTask`
+  (`flowable:type="dmn"`, `decisionTableReferenceKey=diploma-eligibility`) runs it between
+  `OpenZaakAanmaken` and `Beoordelen`, and an exclusive gateway sends `CBGV_ADVIES` through a new
+  `CBGVAdvies` user task before `Beoordelen`, `DIRECT` straight there.
+- **The domain carries the input, not the decision.** The `Registration` aggregate gains a
+  `DiplomaOrigin` (Binnenlands/Buitenlands); `SubmitRegistration` passes it to
+  `StartRegistrationProcessAsync`, which sets it as the `diplomaOrigin` start variable. The domain
+  never evaluates the DMN and never learns the route — that is the process's concern.
+- **Deployed like the BPMN.** The DMN is version-controlled in `workflows/` and deployed to the DMN
+  engine by the same `flowable-init` step (via `dmn-api/dmn-repository/deployments`), staged into the
+  `fl-bpmn` volume alongside the BPMN.
+
+## Consequences
+
+**Positive**
+
+- The eligibility rule is a first-class, inspectable workflow artefact (matching the PRD's BPMN+DMN
+  framing); business users can read/adjust the decision table without touching domain code.
+- §8.2 stays clean: the Workflow Client remains the only code talking to Flowable, and the decision
+  runs inside the process the client already started — no domain→Flowable round-trip for a decision.
+- The domain change is minimal and additive: one value on the aggregate, one start variable.
+
+**Negative / costs**
+
+- Deviates from #14's literal "evaluated by the Domain Service via Workflow Client" wording (noted on
+  the issue). The outcome — DMN decides eligibility, foreign diplomas get the CBGV step — is unchanged.
+- The DMN and its `businessRuleTask` wiring are validated live (verify-domain drives a foreign
+  registration through CBGV-advies and a domestic one straight to beoordeling, exercising both
+  branches), not in unit tests — consistent with ADR-0009/0014/0015. The domain unit/acceptance tests
+  cover only that the origin is carried into the process.
+
+## Alternatives considered
+
+- **Domain evaluates the DMN via the Workflow Client** (the issue's wording). Rejected: it couples
+  the domain to Flowable for a decision and splits the routing across two places (domain computes,
+  BPMN branches), for no benefit over letting the engine that owns the process own the decision.
+- **Eligibility rules in domain C#.** Rejected: it moves a governable business decision out of the
+  DMN the PRD calls for, and hard-codes what the reference app is meant to demonstrate as data.