feat: ChainRule condition steps — rule-based final hops for chains

A chain step of the form { rule: <config>, conditionStep: true } is a
condition-gated hop instead of an edge traversal. It is valid only as the
FINAL step: the object is known, so the engine verifies the referenced rule
at (intermediate, object) for each current path. Enables a chain's object-side
hop to reference a defeasible/logical evidence (e.g.
member_of(user,*g){ gated(g,doc) } where gated is WHEN/UNLESS).

- RuleEvaluator wires the ChainRule with itself so condition steps can
  evaluate sub-rules recursively.
- ChainRule constructor accepts the ruleEvaluator; the traversal loop handles
  a final condition step (min-combined possibility, reliability product,
  threshold respect). Non-final condition steps return condition_step_not_final.
- The rule config is part of the chain cache key (JSON.stringify of steps).

Tests: chain-condition-step (grant/deny/missing-edge/non-final/parallel-max).

package.json

@@ -1,6 +1,6 @@
 {
   "name": "@arbiter/core",
-  "version": "1.0.2",
+  "version": "1.0.3",
   "description": "Arbiter core engine: graph indices, relation/reachability, authorization rule evaluator, DSL/AST, condensed & sharded snapshots, and evidence fusion.",
   "license": "ISC",
   "author": "",