refactor(auth): land Session -> Principal, add MedewerkerAdapter (RB-13)

ADR-0002 SS3 models Zorgverlener/Medewerker as different Principal
variants with different login flows. Actor #2 (apps/behandelportal)
landed in WP-61/67 and the union never followed: grep -rn "Principal"
returned one hit, a comment. Both apps' auth/domain/session.ts stayed
byte-identical (`{ bsn, naam }`), so the backoffice's Behandelaar
carried a BSN and logged into the backoffice as a citizen, by DigiD,
under a fabricated citizen's name (login.page.ts). The divergence
ADR-0002 predicted took an orthogonal side door instead
(medewerker.interceptor.ts's X-Medewerker/X-Rollen stamp, which never
touches SessionStore) -- which is why ssp/auth and bhp/auth still
measured as 100%/84% duplicated after ADR-C-006 shared the route
guards. RB-09 (landed the day before) made the backend's
IIdentityProvider able to say "no identity" and fail closed; this
ticket is its named FE half.

Each app's auth/domain/session.ts becomes principal.ts, holding the
one Principal variant that app actually has an actor for: ssp keeps
`{ kind: 'zorgverlener', bsn, naam }` (G1 still strips the BSN before
persisting); behandelportal gets `{ kind: 'medewerker', medewerkerId,
naam, rollen }` (no BSN to strip -- G2 shape validation only). A new
MedewerkerAdapter replaces DigidAdapter in behandelportal, resolving
the existing MEDEWERKER_ID/currentRollen() dev stand-in into a
Principal; because there is no credential to check, it returns
Principal directly rather than a Result whose error variant could
never occur. login.page.ts stops being a BSN/wachtwoord form -- one
explainer line and an "Inloggen met SSO" button -- and its dead
error-handling branch goes with the Result wrapper that justified it.

Measured with tools/baseline-scan.mjs --dup: auth duplication drops
from 168/168 (ssp) and 168/200 (bhp) to 32/179 and 32/259 -- under the
backlog's <40 target. What remains is the ADR-C-006 route-guard
re-export (deliberately identical), generic test/story-file
boilerplate, and one shared fragment of the root-singleton-store
idiom -- not re-converged identity or login-flow logic. SS3's
prediction that the two actors would authenticate differently enough
to justify not sharing auth has now actually been tested, not just
asserted, and held.

Also: renamed Session.bsn to Principal.bsn in two doc comments
(libs/shared/src/infrastructure/subject.ts, subject.interceptor.ts)
that cited the old type name; regenerated
libs/shared/docs/behaviour-spec.mdx (generated file, per its own
banner); recorded the resolution in ADR-0002 as a new amendment,
replacing its "Known debt" section.

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

apps/ssp/src/app/auth/domain/principal.ts

@@ -0,0 +1,39 @@
+/**
+ * Who is logged in. Framework-free domain type.
+ *
+ * The `zorgverlener` variant of ADR-0002 §3's `Principal` union — the SSP has exactly
+ * one actor kind (a citizen, authenticated via DigiD/BSN), so this app's own copy of
+ * the union only ever holds this one member. `kind` is still a discriminant, not
+ * decoration: it is what makes `apps/behandelportal`'s `medewerker` variant a
+ * genuinely different type rather than a same-shaped coincidence, and what a future
+ * third actor (§4 — admin/auditor/institution-rep) would add a member to.
+ */
+export interface Principal {
+  readonly kind: 'zorgverlener';
+  readonly bsn: string;
+  readonly naam: string;
+}
+
+export function isAuthenticated(p: Principal | null): p is Principal {
+  return p !== null;
+}
+
+/**
+ * Parse a persisted principal out of a raw `localStorage` string (best-effort;
+ * anything that isn't a well-shaped record → logged out). G2: validate the
+ * shape before trusting it. G1: even if a stored entry carries a `bsn`, the
+ * restored principal's `bsn` is always `''` — the BSN is never persisted (see
+ * the `SessionStore` effect that writes it), so a legacy or tampered entry
+ * cannot resurrect one.
+ */
+export function parseStoredPrincipal(raw: string | null): Principal | null {
+  try {
+    if (!raw) return null;
+    const parsed = JSON.parse(raw) as Partial<Principal>;
+    return typeof parsed?.naam === 'string'
+      ? { kind: 'zorgverlener', bsn: '', naam: parsed.naam }
+      : null;
+  } catch {
+    return null;
+  }
+}