feat: intermediate chain condition steps, graph-version cache invalidation, rolling-hash chain keys; fix vacuous rigor invariants

Chain intermediates (rule-based reachability):
- ChainRule: a condition step ({ rule, conditionStep }) at an INTERMEDIATE
  position is now EXPANDED from the current node — the rule's base edges'
  destinations, filtered by its defeaters/requirements — and traversal
  continues from each discovered node. Adds _expandRuleFromSrc / direct /
  logical(union/intersection) / defeasible / nested-chain expansion.
- RuleEvaluator: _subjectIsObject flag for unary predicate calls whose subject
  entity IS the object parameter (trusted(other) inside peer_trusted(user,
  other)); previously only subject-var unary calls (_subjectAsObject) were
  handled, so object-var unary defeaters never fired.

Graph-version cache invalidation:
- Arbiter gains a monotonic _graphVersion, incremented on every relation
  mutation. ChainRule result cache, RuleEvaluator rule-result cache, and
  DecisionCache rule cache now stamp entries with the graph version and treat
  any mismatch as a miss — graph mutations can no longer serve stale
  chain/authorization results.

Rolling-hash cache keys:
- UnifiedKeyManager.createChainKey now builds a 53-bit rolling hash (dual
  FNV-1a lanes, exact for ints/floats/strings/nested configs) instead of
  JSON.stringify — no string allocation or serialization on the chain-cache
  hot path. Composite keys stay structured strings because the direct-check
  cache pattern-invalidates by relation ID.

Rigor invariant migration (correctness):
- All 43 rigor test files' throw-based invariants ({ error, errorMessage } =>
  !error && !errorMessage) never saw fn throws — vacuous. Migrated to
  ({ actual }) => actual !== undefined, which fails on any thrown violation
  while passing legitimate null-skips. The migration immediately surfaced
  two latent bugs, now fixed:
    * node-manager/graph-indices skip paths returned bare undefined (falsy
      sentinel) — return { skipped: true }.
    * complex-graph-values-crucible expiry section rewrote values equal to the
      mutation loop's last write; the engine (by design) keeps the old
      timestamp on same-value rewrites so the pre-expiry grant never
      materialized. Now writes guaranteed-different values.

tests/rigor/challenge-proof.test.js

@@ -108,7 +108,7 @@ describe('PartialGraphContext.getChallengeProof (rigor)', () => {
       rigor.crucible([
         rigor.invariant(
           'oracle-matches-brute-force',
-          ({ error, errorMessage }) => !error && !errorMessage
+          ({ actual }) => actual !== undefined
         )
       ])
     ).run({ seed: 'challenge-proof-oracle', effort: 1500 , artifacts: { dir: '', persist: 'never' }});
@@ -150,7 +150,7 @@ describe('PartialGraphContext.getChallengeProof (rigor)', () => {
         )
       ],
       rigor.crucible([
-        rigor.invariant('no-expired', ({ error, errorMessage }) => !error && !errorMessage)
+        rigor.invariant('no-expired', ({ actual }) => actual !== undefined)
       ])
     ).run({ seed: 'challenge-proof-expired', effort: 800 , artifacts: { dir: '', persist: 'never' }});
 
@@ -225,7 +225,7 @@ describe('PartialGraphContext.getChallengeProof (rigor)', () => {
         )
       ],
       rigor.crucible([
-        rigor.invariant('most-recent', ({ error, errorMessage }) => !error && !errorMessage)
+        rigor.invariant('most-recent', ({ actual }) => actual !== undefined)
       ])
     ).run({ seed: 'challenge-proof-most-recent', effort: 800 , artifacts: { dir: '', persist: 'never' }});
 
@@ -281,7 +281,7 @@ describe('PartialGraphContext.getChallengeProof (rigor)', () => {
         )
       ],
       rigor.crucible([
-        rigor.invariant('within-window', ({ error, errorMessage }) => !error && !errorMessage)
+        rigor.invariant('within-window', ({ actual }) => actual !== undefined)
       ])
     ).run({ seed: 'challenge-proof-within-ms', effort: 1500 , artifacts: { dir: '', persist: 'never' }});