feat(showcase): PII section — mask + elfproef parse (WP-42, mask/parse half)

Add a "PII — maskeren & parsen" section to /concepts demonstrating the WP-40 pieces
with FP + atomic design, framed for AVG art. 9 / data-minimisation: a live
<app-masked-value> atom (masked BSN that reveals on click; real reveal is step-up +
audited in behandel-scherm) and a live parseBsn elfproef parse. Both show the real
linked source via the WP-39 snippet mechanism (new showcase regions in bsn.ts + pii.ts,
registered in gen-snippets.mjs). Delivers WP-42's showcase demo; the persisted-audit
half stays pending WP-41.

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

scripts/gen-snippets.mjs

@@ -14,6 +14,8 @@ const SOURCES = [
   'src/app/registratie/domain/value-objects/postcode.ts',
   'src/app/registratie/domain/change-request.machine.ts',
   'src/app/herregistratie/domain/intake.machine.ts',
+  'src/app/shared/kernel/bsn.ts',
+  'src/app/shared/kernel/pii.ts',
 ];
 
 const START = /\/\/\s*#region showcase:([\w-]+)/;