feat(stamdata): extract policy-question text into Stamdata

Move the geldigheidsvragen wording out of DiplomaRules into
Stamdata.PolicyQuestions (business-editable text, config-as-code); DiplomaRules
keeps only the rule of which questions apply. Extend StamdataValidationTests
(no blank id/wording, distinct ids in the manual set) and update ADR-0004.

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

docs/reference/architecture/0004-stamdata-as-code.md

@@ -75,7 +75,8 @@ everyone. Stamdata (the rules and reference tables the whole register runs on) s
 - **−** A change needs the PR pipeline — not instant, and a non-developer may need dev
   assistance to edit C# (mitigated later by a low-code editor that emits a PR, or by a
   data-file format if hand-editing ergonomics ever outweigh maximal compile-time safety).
-- **Pilot shipped with this ADR:** the profession↔diploma map extracted to
-  `Stamdata/Professions.cs`, `DiplomaRules` refactored to consume it (behaviour unchanged),
-  and `StamdataValidationTests` added. Policy-question text and document-category
-  definitions follow the same pattern as obvious next steps; not moved yet.
+- **Shipped with this ADR:** the profession↔diploma map (`Stamdata/Professions.cs`) and the
+  policy-question wording (`Stamdata/PolicyQuestions.cs`) extracted from `DiplomaRules`,
+  which now consumes both (behaviour unchanged), guarded by `StamdataValidationTests`.
+  Document-category definitions follow the same pattern as the obvious next step; not moved
+  yet.