feat: tuple_to_userset intermediate reachability; ratio-based benchmark gate

- ChainRule._expandRuleFromSrc now expands tuple_to_userset configs: from a
  source node the reachable set is the objects sharing an intermediate with
  the source (src ->computed-> intermediate ->tupleset-> object, direction
  aware, weakest-link combined). Lets a TTU evidence serve as an intermediate
  condition step in a chain.
- scripts/benchmark.js: ratio-based self-calibration. Comparing each action's
  RATIO to a cheap reference action (default check[direct-hit]) cancels
  machine-load swings that scale all actions proportionally, so the gate only
  fails on code regressions that shift a single action's ratio. The reference
  is still checked absolutely with a loose bound. Verified: stable across
  runs, and a simulated union-ttu slowdown is caught (+76.8% ratio).

Tests: chain-condition-step intermediate TTU expansion.

package.json

@@ -1,6 +1,6 @@
 {
   "name": "@arbiter/core",
-  "version": "1.0.4",
+  "version": "1.0.5",
   "description": "Arbiter core engine: graph indices, relation/reachability, authorization rule evaluator, DSL/AST, condensed & sharded snapshots, and evidence fusion.",
   "license": "ISC",
   "author": "",