docs: close WP-72..75, regenerate behaviour spec

Four close-outs and their README rows. The behaviour spec is regenerated
once here rather than per-track — it derives from every test name in the
repo, so any track running it would have conflicted with the other three.

Records two findings the arc surfaced but did not cause: the /brief/preview
staleness for non-DemoOwner identities (blocking per-spec identity isolation
in brief-v2.spec.ts), and that WP-72/73 had to share a commit because both
edit Program.cs — separate execution waves prevented build collisions but
did not produce separable diffs.

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

docs/project/backlog/README.md

@@ -122,6 +122,10 @@ for its existing violations, so every WP ends green.
 | [WP-69](WP-69-intake-scholing-threshold-enforcement.md) | Enforce the scholing threshold server-side                                         | 12 · DDD hardening          | done   |
 | [WP-70](WP-70-test-data-builders.md)                    | Test-data builders: illegal fixtures unrepresentable (ADR-0006)                    | 12 · DDD hardening          | done   |
 | [WP-71](WP-71-test-framework-coherence.md)              | Test framework coherence: BDD/DDD alignment, close the escape hatches              | 12 · DDD hardening          | done   |
+| [WP-72](WP-72-delete-legacy-submit-endpoints.md)        | Delete the dead legacy submit endpoints                                            | 12 · DDD hardening          | done   |
+| [WP-73](WP-73-domain-unions.md)                         | `RegistrationStatus` and `Aanvraag` as closed unions                               | 12 · DDD hardening          | done   |
+| [WP-74](WP-74-e2e-isolation.md)                         | E2E isolation without a new backend endpoint                                       | 12 · DDD hardening          | done   |
+| [WP-75](WP-75-fe-be-seam-closure.md)                    | Close the remaining FE/BE seams                                                    | 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