docs: ADR-0006 test-data builders, close out WP-70

Writes up the principle behind WP-70's three tracks ("build test data
through the same door production code uses") as ADR-0006, with a decision
table for which fixture idiom fits which test type. Updates the
test-strategy skill (adds the Fixtures rule, fixes its stale pre-monorepo
src/app/... worked-example paths) and the shared Storybook testing.mdx page
to match. Closes WP-70 with the signatures/counts as actually shipped.

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

docs/project/backlog/README.md

@@ -120,6 +120,7 @@ for its existing violations, so every WP ends green.
 | [WP-67](WP-67-monorepo-behandelportal.md)               | Merge behandelportal into this repo as a monorepo                                  | 11 · Behandelportal         | done   |
 | [WP-68](WP-68-ddd-aggregate-hardening.md)               | Aggregate invariants + status modelling (architecture review)                      | 12 · DDD hardening          | done   |
 | [WP-69](WP-69-intake-scholing-threshold-enforcement.md) | Enforce the scholing threshold server-side                                         | 12 · DDD hardening          | todo   |
+| [WP-70](WP-70-test-data-builders.md)                    | Test-data builders: illegal fixtures unrepresentable (ADR-0006)                    | 12 · DDD hardening          | done   |
 
 Sequencing dependencies (stated in the WPs too): 01 before 10–15 (axe covers story churn);
 03/04 before 05–09 (boundaries stop new violations during refactors); 06 before 07 (typed