fix(relay-auth): close 5 pre-production security findings (F1–F5)

F1 (HIGH): bind step-up freshness to the required method (stepUpMethod) so a
  login-time or weaker (TOTP) factor can no longer satisfy a passkey requirement.
F2 (HIGH): make the step-up gate host-driven and fail-closed on connect+reattach
  (deny when policy.required and the principal is missing/stale/wrong-method).
F3 (MED): bind the WebAuthn assertion to the stored credential (credentialId
  cross-check + forward publicKey/credentialId to the verifier).
F4 (LOW): make verifyDpopProof totally fail-safe (no unhandled throw), wrap the
  authz boundary into a clean audited 401, and validate cnfJkt format at issue.
F5 (LOW): return newSignCount from finishAuthentication so the caller can persist
  the advanced counter (WebAuthn clone detection).

All fixes ship regression tests. relay-auth: 122 tests green, tsc clean.

relay-auth/test/types.test.ts

@@ -0,0 +1,60 @@
+import { describe, it, expect } from 'vitest'
+import {
+  AuthenticatedPrincipalSchema,
+  StepUpPolicySchema,
+  type AuthenticatedPrincipal,
+  type StepUpPolicy,
+} from '../src/types.js'
+
+const basePrincipal = {
+  kind: 'human' as const,
+  accountId: 'acct-A',
+  principalId: 'cred-1',
+  amr: ['passkey' as const],
+  authAt: 1_700_000_000,
+  stepUpAt: null,
+}
+
+describe('StepUpPolicy.required (host-driven, fail-closed)', () => {
+  it('parses a policy with required=true', () => {
+    const policy: StepUpPolicy = { required: true, maxAgeSeconds: 300, requiredMethod: 'passkey' }
+    expect(StepUpPolicySchema.parse(policy)).toEqual(policy)
+  })
+
+  it('parses a policy with required=false', () => {
+    const policy: StepUpPolicy = { required: false, maxAgeSeconds: 300, requiredMethod: 'passkey' }
+    expect(StepUpPolicySchema.parse(policy)).toEqual(policy)
+  })
+
+  it('rejects a policy missing required', () => {
+    const bad = { maxAgeSeconds: 300, requiredMethod: 'passkey' }
+    expect(StepUpPolicySchema.safeParse(bad).success).toBe(false)
+  })
+
+  it('rejects a non-boolean required', () => {
+    const bad = { required: 'yes', maxAgeSeconds: 300, requiredMethod: 'passkey' }
+    expect(StepUpPolicySchema.safeParse(bad).success).toBe(false)
+  })
+})
+
+describe('AuthenticatedPrincipal.stepUpMethod (F1 method-binding, optional)', () => {
+  it('parses a principal that omits stepUpMethod (backward compatible)', () => {
+    const parsed = AuthenticatedPrincipalSchema.parse(basePrincipal)
+    expect(parsed.stepUpMethod).toBeUndefined()
+  })
+
+  it('parses a principal with an explicit stepUpMethod', () => {
+    const p: AuthenticatedPrincipal = { ...basePrincipal, stepUpAt: 1_700_000_100, stepUpMethod: 'passkey' }
+    expect(AuthenticatedPrincipalSchema.parse(p).stepUpMethod).toBe('passkey')
+  })
+
+  it('parses a principal with stepUpMethod=null (no step-up)', () => {
+    const parsed = AuthenticatedPrincipalSchema.parse({ ...basePrincipal, stepUpMethod: null })
+    expect(parsed.stepUpMethod).toBeNull()
+  })
+
+  it('rejects an invalid stepUpMethod value', () => {
+    const bad = { ...basePrincipal, stepUpMethod: 'sms' }
+    expect(AuthenticatedPrincipalSchema.safeParse(bad).success).toBe(false)
+  })
+})