feat(stamdata): profession↔diploma map as validated config-as-code

Extract the profession↔diploma table out of DiplomaRules into a dedicated
Stamdata.Professions module (business-editable data, separated from the rules
that consume it) and add StamdataValidationTests as the build-time gate: every
seeded diploma program must resolve to a real profession, no blank entries. A
bad edit now fails the build instead of silently rendering "Onbekend". Rules
and behaviour unchanged.

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

backend/src/BigRegister.Api/Domain/Diplomas/DiplomaRules.cs

@@ -1,29 +1,23 @@
+using BigRegister.Stamdata;
+
 namespace BigRegister.Domain.Diplomas;
 
 /// <summary>
-/// SERVER-OWNED business rules for diplomas. This is the single place a policy
-/// changes: which profession a study program maps to, and which policy questions
-/// (geldigheidsvragen) apply to a diploma. The frontend renders these; it never
-/// derives them.
+/// SERVER-OWNED business rules for diplomas: which profession a study program maps to,
+/// and which policy questions (geldigheidsvragen) apply to a diploma. The frontend
+/// renders these; it never derives them.
+///
+/// The profession↔program *data* is business-editable stamdata in
+/// <see cref="Professions"/> (config-as-code, ADR-0004); the *rules* below consume it.
 /// </summary>
 public static class DiplomaRules
 {
-  // RULE: study program → BIG profession.
-  private static readonly Dictionary<string, string> ProfessionByProgram = new(StringComparer.OrdinalIgnoreCase)
-  {
-    ["geneeskunde"] = "Arts",
-    ["verpleegkunde"] = "Verpleegkundige",
-    ["fysiotherapie"] = "Fysiotherapeut",
-    ["farmacie"] = "Apotheker",
-    ["tandheelkunde"] = "Tandarts",
-  };
-
+  // RULE: study program → BIG profession (data lives in Stamdata.Professions).
   public static string ProfessionFor(Diploma d) =>
-      ProfessionByProgram.TryGetValue(d.Opleiding, out var beroep) ? beroep : "Onbekend";
+      Professions.ByProgram.TryGetValue(d.Opleiding, out var beroep) ? beroep : "Onbekend";
 
   /// <summary>Professions a user may declare for a manual (unlisted) diploma.</summary>
-  public static IReadOnlyList<string> ManualProfessions() =>
-      ProfessionByProgram.Values.Distinct().ToList();
+  public static IReadOnlyList<string> ManualProfessions() => Professions.All();
 
   // --- Policy questions (geldigheidsvragen) ---
 

backend/src/BigRegister.Api/Stamdata/Professions.cs

@@ -0,0 +1,32 @@
+namespace BigRegister.Stamdata;
+
+/// <summary>
+/// BUSINESS-EDITABLE STAMDATA (config-as-code). Which BIG profession (beroep) each
+/// study program (opleiding) maps to. This is the one table the business tunes when
+/// a program starts or stops leading to a registered profession.
+///
+/// Change it by editing this file and opening a PR — NOT via a production database.
+/// The C# compiler catches shape/type mistakes; <c>StamdataValidationTests</c> catches
+/// the referential integrity it can't (e.g. a seeded diploma whose program has no
+/// profession here). So a bad edit fails the build, never prod. See ADR-0004
+/// (docs/reference/architecture/0004-stamdata-as-code.md).
+///
+/// This is DATA, not logic: the rules that consume it (which questions a diploma needs,
+/// how a manual diploma is treated) stay in <c>DiplomaRules</c>.
+/// </summary>
+public static class Professions
+{
+  public static readonly IReadOnlyDictionary<string, string> ByProgram =
+      new Dictionary<string, string>(StringComparer.OrdinalIgnoreCase)
+      {
+        ["geneeskunde"] = "Arts",
+        ["verpleegkunde"] = "Verpleegkundige",
+        ["fysiotherapie"] = "Fysiotherapeut",
+        ["farmacie"] = "Apotheker",
+        ["tandheelkunde"] = "Tandarts",
+      };
+
+  /// <summary>Distinct professions, in declaration order — the list a user may declare
+  /// for a manual (unlisted) diploma.</summary>
+  public static IReadOnlyList<string> All() => ByProgram.Values.Distinct().ToList();
+}