test(backend): split RuleTests.cs by aggregate, refresh DDD doc (WP-71)

RuleTests.cs held five aggregates' rules as nested classes in one file,
misaligned with Domain/<Aggregate>/ and with the Acceptance/Builders/
folder convention WP-70 started. Split into Domain/<Aggregate>RuleTests.cs
(pure move — same names, same bodies, same count) plus a new
ApplicationRuleTests.cs (the enum invariant moved out of the
WebApplicationFactory-booting ApplicationTests.cs, since it's a pure
Enum.GetNames check with no business needing a web host) and
OrgTemplateRuleTests.cs (RejectDraft had no direct unit test before,
only endpoint coverage).

libs/shared/docs/layers.mdx still taught the pre-WP-67 shape (six
contexts, no apps/libs split, enforcement via ESLint) — updated to the
real monorepo structure and to dependency-cruiser as the actual
enforcement mechanism. Adds specs for registration.policy.ts's
isStatusConsistent (untested; its backend mirror is) and both apps'
auth/domain/session.ts.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/tests/BigRegister.Tests/Domain/DiplomaRuleTests.cs

@@ -0,0 +1,42 @@
+using BigRegister.Domain.Diplomas;
+
+namespace BigRegister.Tests.Domain;
+
+public class DiplomaRuleTests
+{
+  private static Diploma Diploma(string opleiding, bool engelstalig) =>
+      new("x", "naam", "instelling", 2011, opleiding, engelstalig);
+
+  [Theory]
+  [InlineData("geneeskunde", "Arts")]
+  [InlineData("verpleegkunde", "Verpleegkundige")]
+  [InlineData("onbekend-programma", "Onbekend")]
+  public void Profession_is_derived_from_program(string opleiding, string expected) =>
+      Assert.Equal(expected, DiplomaRules.ProfessionFor(Diploma(opleiding, false)));
+
+  [Fact]
+  public void English_diploma_requires_dutch_proficiency()
+  {
+    var questions = DiplomaRules.QuestionsFor(Diploma("geneeskunde", engelstalig: true));
+    Assert.Single(questions);
+    Assert.Equal("nl-taalvaardigheid", questions[0].Id);
+  }
+
+  [Fact]
+  public void Dutch_diploma_has_no_policy_questions() =>
+      Assert.Empty(DiplomaRules.QuestionsFor(Diploma("geneeskunde", engelstalig: false)));
+
+  [Fact]
+  public void Manual_diploma_gets_maximal_set()
+  {
+    var questions = DiplomaRules.ManualQuestions();
+    Assert.Equal(3, questions.Count);
+    Assert.Equal(new[] { "nl-taalvaardigheid", "diploma-erkend", "toelichting" },
+        questions.Select(q => q.Id));
+  }
+
+  [Fact]
+  public void Manual_professions_match_known_programs() =>
+      Assert.Equal(new[] { "Arts", "Verpleegkundige", "Fysiotherapeut", "Apotheker", "Tandarts" },
+          DiplomaRules.ManualProfessions());
+}