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.

tests/rigor/dsl-illegal-mutations.test.js

@@ -96,6 +96,14 @@ const MUTATIONS = {
       'evidence can_read(user: Employee, doc: Doc) { owns(user, doc) }',
       'fact can_read(user: Employee, doc: Doc)\n  evidence can_read(user: Employee, doc: Doc) { owns(user, doc) }'
     )
+  },
+  non_lowerable_chain_step: {
+    desc: 'a defeasible evidence used as a chain step (cannot lower to an edge)',
+    mustFail: true,
+    apply: () => VALID_DSL.replace(
+      'evidence can_enter(user: Employee, doc: Doc) { member_of(user, *g) { can_access(g, doc) } }',
+      'evidence can_gated(group: Group, doc: Doc) { WHEN can_access(group, doc) UNLESS banned(group) }\n  evidence can_enter(user: Employee, doc: Doc) { member_of(user, *g) { can_gated(g, doc) } }'
+    ).replace('fact can_access(group: Group, doc: Doc)', 'fact can_access(group: Group, doc: Doc)\n  fact banned(group: Group)')
   }
 };