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/tests/BigRegister.Tests/StamdataValidationTests.cs

@@ -0,0 +1,41 @@
+using BigRegister.Api.Data;
+using BigRegister.Domain.Diplomas;
+using BigRegister.Stamdata;
+
+namespace BigRegister.Tests;
+
+/// <summary>
+/// The compile-time gate for business-editable stamdata (ADR-0004). The C# compiler
+/// already catches shape/type mistakes; these tests catch the referential integrity it
+/// can't, so a bad config edit fails the build instead of reaching production.
+/// </summary>
+public class StamdataValidationTests
+{
+  [Fact]
+  public void Every_seeded_diploma_program_maps_to_a_known_profession()
+  {
+    // The dangling-reference guard: a seed program with no entry in Professions would
+    // silently render "Onbekend" to the user. Fail the build instead.
+    foreach (var d in SeedData.Diplomas)
+      Assert.True(DiplomaRules.ProfessionFor(d) != "Onbekend",
+          $"Diploma program '{d.Opleiding}' has no profession in Stamdata.Professions.");
+  }
+
+  [Fact]
+  public void Profession_map_has_no_blank_programs_or_professions()
+  {
+    Assert.All(Professions.ByProgram, kv =>
+    {
+      Assert.False(string.IsNullOrWhiteSpace(kv.Key), "A profession-map program key is blank.");
+      Assert.False(string.IsNullOrWhiteSpace(kv.Value), $"Program '{kv.Key}' maps to a blank profession.");
+    });
+  }
+
+  [Fact]
+  public void Manual_professions_are_non_empty_and_distinct()
+  {
+    var professions = DiplomaRules.ManualProfessions();
+    Assert.NotEmpty(professions);
+    Assert.Equal(professions.Count, professions.Distinct().Count());
+  }
+}