feat: chain-step evidence composition — expand evidence steps in chains

A chain step that references a derived evidence is now expanded at compile
time, keeping the engine a flat edge-traversal evaluator:

- DIRECT evidence step -> renamed to its underlying relation
  (member_of(user,*g){ group_read(g,doc) } where group_read = can_view
  becomes step 'can_view').
- CHAIN evidence step -> its steps are spliced into the parent chain
  (a sub-path flattens into the linear source->...->object traversal).
- Any other evidence type (defeasible/logical/comparator) as a step is a
  compile-time error: it is a condition, not an edge traversal.
- Cycles and self-references through chain steps are compile-time errors
  (the existing composition cycle guard now covers steps).

Rigor: oracle campaign gains a chain_step_composition construct; illegal
mutations gain a non-lowerable-chain-step case. Fixture suites updated to
retarget the self-recursive 'canRead/canAccess/...' terminals (an unsupported
recursion pattern that now fails loudly) to an any-typed 'reachable' fact,
preserving the nested-pattern parsing intent.

package.json

@@ -1,6 +1,6 @@
 {
   "name": "@arbiter/evidence-dsl",
-  "version": "1.2.0",
+  "version": "1.3.0",
   "description": "Evidence DSL v2 compiler: translates the natural Evidence DSL (ADR-000) into @arbiter/core relation configurations.",
   "license": "ISC",
   "type": "module",