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/traversal-edge-cases.test.js

@@ -0,0 +1,34 @@
+import { describe, test } from 'node:test';
+import assert from 'node:assert/strict';
+import { Arbiter } from '../../src/core/Arbiter.js';
+
+describe('Traversal edge cases', () => {
+  test('shortestPathLength returns Infinity for missing nodes', () => {
+    const arbiter = new Arbiter();
+    arbiter.addNode('a', 'node');
+    assert.strictEqual(arbiter.traversal.shortestPathLength('a', 'missing'), Infinity);
+  });
+
+  test('shortestPathLength finds a two-hop path', () => {
+    const arbiter = new Arbiter();
+    arbiter.addNode('a', 'node');
+    arbiter.addNode('b', 'node');
+    arbiter.addNode('c', 'node');
+    arbiter.addRelation('a', 'links', 'b');
+    arbiter.addRelation('b', 'links', 'c');
+
+    assert.strictEqual(arbiter.traversal.shortestPathLength('a', 'c'), 2);
+  });
+
+  test('walk returns empty path for unknown start', () => {
+    const arbiter = new Arbiter();
+    assert.deepStrictEqual(arbiter.traversal.walk('missing', 3), []);
+  });
+
+  test('walk stops when no neighbors are present', () => {
+    const arbiter = new Arbiter();
+    arbiter.addNode('solo', 'node');
+    const path = arbiter.traversal.walk('solo', 5);
+    assert.deepStrictEqual(path, ['solo']);
+  });
+});