initial commit: @arbiter/core authorization engine with js-rigor hardening

Zanzibar-style authorization graph engine (direct/chain/TTU/defeasible/
binary modes, condensed snapshots, value relations) with 39 rigor test
campaigns. Includes fixes for snapshot binary writer/reader format
mismatch (snapshot-of-snapshot corruption), possibility write-boundary
validation, empty-graph snapshot serialization, relation lookup cache
direction collision, config-redefinition cache invalidation, binary
threshold semantics, defeasible compiled routing, and comparator
reason whitelisting.

tests/engine/fast-check-node-ids.test.js

@@ -0,0 +1,28 @@
+import { describe, test } from 'node:test';
+import assert from 'node:assert/strict';
+import fc from 'fast-check';
+import { Arbiter } from '../../src/core/Arbiter.js';
+
+describe('Fast-check: node id mapping invariants', () => {
+  test('resolveNodeId/resolveKey round-trip', () => {
+    fc.assert(
+      fc.property(
+        fc.set(fc.string({ minLength: 1, maxLength: 12 }), { minLength: 1, maxLength: 20 }),
+        (keys) => {
+          const arbiter = new Arbiter();
+          for (const key of keys) {
+            arbiter.addNode(key, 'generic');
+          }
+
+          for (const key of keys) {
+            const id = arbiter.resolveNodeId(key);
+            assert.ok(id !== undefined && id !== null);
+            const roundTrip = arbiter.resolveKey(id);
+            assert.strictEqual(roundTrip, key);
+          }
+        }
+      ),
+      { numRuns: 50 }
+    );
+  });
+});