docs(backlog): add Phase 8 — platform/DX/showcase (WP-37..46)

New phase from the 5-item analysis: linked showcase snippets + animations,
runnable scaffolding generators (pages/wizards/context/SSP), dependency graph +
declarative boundaries, PII hardening (branded Bsn VO + masked-value atom +
persisted no-PII audit) doubling as a privacy showcase, and Vitest coverage.
Goal: both a teaching showcase and a production starter template. Sequenced with
priorities + dependencies in the README.

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

docs/project/backlog/WP-40-pii-kernel.md

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+# WP-40 — PII kernel: branded `Bsn` VO + masked-value atom
+
+Status: todo
+Phase: 8 — platform/DX/showcase
+Priority: P2
+
+## Why
+
+Masking is real but ad-hoc: pure helpers live in `debug-state/mask.ts`, the BSN is a bare `string`
+in `session.ts` (no branded VO / checksum, unlike `BigNummer`/`Postcode`), and masked state is
+detected by `.includes('*')` sniffing — no reusable atomic-design component. For a register this is
+AVG art. 9 (BSN = special category) territory; the pieces should be first-class and reusable.
+
+## Decisions
+
+- `Bsn` branded value object + `parseBsn` with the **elfproef** (9 digits, weighted mod-11)
+  checksum — parity with the other value objects. Replace the bare `string` in `session.ts`.
+- Consolidate the pure maskers into `shared/kernel/pii.ts` (functional core): `maskBsn`, `maskTail`,
+  `redactProfile`, … Keep the backend `MaskTail` twin in sync.
+- `<app-masked-value>` atom (`shared/ui`) — masked by default, optional reveal affordance + a11y;
+  replaces `.includes('*')` sniffing and composes into the behandel-scherm reveal.
+
+## Files
+
+- New `src/app/shared/kernel/bsn.ts` (+spec, elfproef cases) and `shared/kernel/pii.ts` (+spec).
+- `src/app/auth/domain/session.ts` — `bsn: Bsn`.
+- New `src/app/shared/ui/masked-value/` atom (+ story).
+- `src/app/shared/ui/debug-state/mask.ts` — re-export/move to the kernel module.
+
+## Acceptance criteria
+
+- [ ] `parseBsn` accepts valid elfproef numbers, rejects bad checksum/length (spec).
+- [ ] `<app-masked-value>` renders masked by default with an accessible reveal; behandel-scherm uses it.
+- [ ] Pure maskers have specs (no TestBed); backend `MaskTail` parity noted; `npm run ci` green.