test(e2e): shared Actors/SeedRefs/loginAs, kill duplicated magic strings (WP-70)

The demo BSN, password, and DigiD login sequence were copy-pasted verbatim
into all three specs; the diploma id #diploma-d1 was coupled to SeedData.cs's
ordering by comment only, with no compile-time check if the seed shape
changed. e2e/support/actors.ts names both: Actors.zorgverlener + loginAs()
for the login sequence, SeedRefs.diplomaZonderPolicyVragen for the seed
coupling (with the "why d1" reasoning attached to the name, not scattered
across specs). Zero assertions changed — pure extract-and-rename of test
setup. e2e test-isolation (the shared mutable backend) is a documented
follow-up, not fixed here — see ADR-0006.

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

e2e/support/actors.ts

@@ -0,0 +1,48 @@
+import type { Page } from '@playwright/test';
+
+/**
+ * A demo identity the fake DigiD login accepts. Auth is faked (CLAUDE.md) — the
+ * form only ever emits `bsn` on submit, so `wachtwoord` is never actually checked;
+ * it's filled in anyway because the field is marked required in the UI.
+ */
+export interface Actor {
+  readonly bsn: string;
+  readonly wachtwoord: string;
+}
+
+/**
+ * The demo identities seeded by the backend. Today there is exactly one seeded
+ * citizen (`backend/src/BigRegister.Api/Data/SeedData.cs`'s `Person`/`Registration`,
+ * whose BSN also matches `DocumentStore.DemoOwner`) — named for the ROLE it plays
+ * in a spec, not its BSN, so a spec reads as "log in as the zorgverlener", not
+ * "log in as 123456782".
+ */
+export const Actors = {
+  zorgverlener: { bsn: '123456782', wachtwoord: 'demo' },
+} as const satisfies Record<string, Actor>;
+
+/** The shared DigiD-style mock login sequence every e2e spec starts from. */
+export async function loginAs(page: Page, actor: Actor): Promise<void> {
+  await page.goto('/login');
+  await page.getByLabel('BSN').fill(actor.bsn);
+  await page.getByLabel('Wachtwoord').fill(actor.wachtwoord);
+  await page.getByRole('button', { name: 'Inloggen met DigiD' }).click();
+}
+
+/**
+ * Seeded fixtures specs depend on by id, named by
+ * `backend/src/BigRegister.Api/Data/SeedData.cs`'s seed order — reseeding that
+ * file in a different order silently breaks these with no compile error, so give
+ * the raw id a name instead of leaving it a bare literal in each spec.
+ */
+export const SeedRefs = {
+  /**
+   * The first DUO diploma (id "d1": Geneeskunde, Universiteit Leiden, non-English).
+   * Chosen deliberately, not arbitrarily: it's the one seeded diploma that carries
+   * zero policy questions, so it drives the wizard down its minimum required path
+   * — the only required upload is identiteit.
+   */
+  diplomaZonderPolicyVragen: 'd1',
+  /** The seeded registration's BIG-nummer (`SeedData.Registration`). */
+  bigNummer: '19012345601',
+} as const;