fix(authz): fix perf, doc, and fail-open gaps found in second review

Re-review of Task 6's fix round 1 (plan amendment d6a2d054) found three
items in that diff plus one adjacent pre-existing issue that C1 made
reachable:

- Important (perf): permissionsFor() rebuilt the deny Set on every
  entry in the granted set (O(grants x denies) allocations on a
  per-request path). Hoisted to build the Set once. Measured
  4000x4000: 665.92ms before, 3.90ms after.
- Important (contract accuracy): permissionsFor() only half-agrees
  with decide() — a narrow deny under a broad grant (e.g. role editor's
  "post:*" plus a deny on "post:delete") can't be represented in a flat
  Set, so the set still contains "post:*" while decide() correctly
  refuses "post:delete". Documented as NOT authoritative on the
  AuthzResolver interface, and pinned with a regression test asserting
  the divergence is deliberate.
- Minor: subject.id === "" was audited as subjectId: "" instead of
  omitted, so consoleAuditSink printed a blank subject= rather than
  subject=anonymous. Reused the same non-empty-string guard as the
  decide() path.
- Important (adjacent, advanced.ts): owner() compared subject[key] to
  resource[key] with Object.is without checking either side was
  present, so two absent ids (Object.is(undefined, undefined) ===
  true) satisfied ownership. Unreachable before this task, but C1 now
  runs bound policies for anonymous/empty subjects, putting this on a
  live path. Fixed to deny whenever either side is undefined or null.

Every fix's regression test was verified by reverting the fix and
confirming the test fails against the pre-fix code before restoring.

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

packages/authz/src/advanced.ts

@@ -39,10 +39,18 @@ export function owner<SubjectType extends Subject, Resource extends Record<strin
   subjectKey: keyof SubjectType = "id",
   resourceKey: keyof Resource | string = "userId",
 ): DecisionPolicy<SubjectType, Resource> {
-  return (subject, resource) =>
-    resource && Object.is(subject[subjectKey], resource[resourceKey as keyof Resource])
+  return (subject, resource) => {
+    const subjectValue = subject?.[subjectKey];
+    const resourceValue = resource?.[resourceKey as keyof Resource];
+    // An absent id on either side must never satisfy ownership.
+    if (subjectValue === undefined || subjectValue === null)
+      return deny("resource ownership required");
+    if (resourceValue === undefined || resourceValue === null)
+      return deny("resource ownership required");
+    return Object.is(subjectValue, resourceValue)
       ? allow("resource owner")
       : deny("resource ownership required");
+  };
 }
 export function anyDecision<S, R>(...policies: DecisionPolicy<S, R>[]): DecisionPolicy<S, R> {
   return async (subject, resource) => {

packages/authz/src/engine.ts

@@ -24,6 +24,16 @@ export interface DecideInput {
 }
 
 export interface AuthzResolver {
+  /**
+   * Effective permissions with denied entries removed — for coarse gating such
+   * as hiding a menu section.
+   *
+   * NOT authoritative. A set of strings cannot express "everything under
+   * `post:*` except `post:delete`", so a narrow deny beneath a broad grant is
+   * not representable here: the set still contains `post:*` while `decide()`
+   * correctly refuses `post:delete`. Gate individual actions with `decide()`
+   * (or `can()` / `filterCan()`), never by matching against this set.
+   */
   permissionsFor(subjectId: string, scope?: AuthzScope): Promise<Set<string>>;
   decide(input: DecideInput): Promise<AuthorizationDecision>;
 }
@@ -85,17 +95,21 @@ export function createAuthzResolver(options: AuthzResolverOptions): AuthzResolve
   const permissionsFor = async (subjectId: string, scope?: AuthzScope): Promise<Set<string>> => {
     const { assignments, granted } = await loadEffective(subjectId, scope);
     if (!assignments.denies.length) return granted;
+    // Hoist the deny set: rebuilding it per entry makes this O(grants x denies)
+    // allocations on a per-request path whose input size an operator controls.
+    const denySet = new Set(assignments.denies);
     const effective = new Set<string>();
     for (const entry of granted) {
-      if (!deniedBy(assignments.denies, entry)) effective.add(entry);
+      if (!permissionMatches(denySet, entry)) effective.add(entry);
     }
     return effective;
   };
 
   const finish = (input: DecideInput, result: AuthorizationDecision): AuthorizationDecision => {
     if (!result.allowed || options.auditAllows) {
+      const rawId = input.subject?.id;
       safeRecord(audit, {
-        subjectId: typeof input.subject?.id === "string" ? input.subject.id : undefined,
+        subjectId: typeof rawId === "string" && rawId !== "" ? rawId : undefined,
         scope: input.scope,
         permission: input.permission,
         allowed: result.allowed,