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/test/engine.test.ts

@@ -290,6 +290,28 @@ describe("createAuthzResolver fail-closed regressions", () => {
     expect(permissionMatches(granted, "post:delete")).toBe(false);
   });
 
+  test("permissionsFor cannot represent a narrow deny under a broad grant (decide remains authoritative)", async () => {
+    // A set of strings can't express "post:* except post:delete": the grant
+    // entry "post:*" survives the subtraction (it isn't itself covered by the
+    // narrower deny "post:delete"), so a set-based check would wrongly say
+    // this permission is available. decide() has no such limitation — it
+    // checks the specific permission against the deny list directly, not
+    // through the granted-entries set — and correctly refuses it. This is a
+    // pinned, deliberate divergence, not a bypass: callers must gate
+    // individual actions with decide()/can(), never by matching this set.
+    const { store, resolver } = make();
+    await store.assignRole("u1", "editor");
+    await store.grant("u1", "post:delete", "deny");
+
+    const granted = await resolver.permissionsFor("u1");
+    expect(granted.has("post:*")).toBe(true);
+    expect(permissionMatches(granted, "post:delete")).toBe(true);
+
+    const decision = await resolver.decide({ subject: { id: "u1" }, permission: "post:delete" });
+    expect(decision.allowed).toBe(false);
+    expect(decision.reason).toMatch(/explicit deny/i);
+  });
+
   test("non-string subject ids deny rather than falling back to anonymous", async () => {
     const { resolver } = make();
     const invalidIds: unknown[] = [0, "", 123, {}];
@@ -302,4 +324,14 @@ describe("createAuthzResolver fail-closed regressions", () => {
       expect(result.reason).toMatch(/invalid subject/i);
     }
   });
+
+  test("an empty-string subject id is not recorded as the audited subjectId", async () => {
+    const { audit, resolver } = make();
+    await resolver.decide({
+      subject: { id: "" } as unknown as { id?: string },
+      permission: "post:read",
+    });
+    expect(audit.events).toHaveLength(1);
+    expect(audit.events[0]!.subjectId).toBeUndefined();
+  });
 });