test(workflow): start carries diploma origin as a process variable (S-13, refs #14)

Add DiplomaOrigin (Binnenlands/Buitenlands) to the Registration aggregate and
submit command, and thread it through the process-start port so the workflow's
DMN can route on it (ADR proposal #100). Failing Workflow Client test asserts the
diplomaOrigin start variable; the client takes the origin but does not emit it yet.

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

services/domain/Big.Domain/DiplomaOrigin.cs

@@ -0,0 +1,17 @@
+namespace Big.Domain;
+
+/// <summary>
+/// Where a zorgprofessional's diploma was issued. It is the input to the diploma-eligibility decision
+/// (S-13): a <see cref="Buitenlands"/> (foreign) diploma routes the registratie through an extra
+/// CBGV-advies assessment step, a <see cref="Binnenlands"/> (domestic) one goes straight to beoordeling.
+/// The decision itself lives in the workflow's DMN, not here (ADR-0016); the domain only carries the
+/// origin and hands it to the process as a start variable.
+/// </summary>
+public enum DiplomaOrigin
+{
+    /// <summary>A Dutch (domestic) diploma. Default for a registration submitted via DigiD.</summary>
+    Binnenlands,
+
+    /// <summary>A foreign diploma (e.g. an eIDAS submission). Triggers the CBGV-advies step.</summary>
+    Buitenlands,
+}