feat: evidence composition — compile-time reference resolution for evidence sub-rules

An evidence may now reference another derived evidence as a sub-rule
(WHEN can_read(user, doc) where can_read is itself an evidence). Resolution
is a compile-time linker pass: after every evidence config is generated, each
direct reference to an evidence is inlined with that evidence's own (resolved)
config, so the engine evaluates a fully-resolved, acyclic config tree.

- resolveEvidenceReferences(): post-generation pass over evidence configs,
  recursing into logical/defeasible containers (when/unless/never/always/
  requires/union/intersection), always.direct nests, and comparator operands.
- Forward references resolve (all configs exist before the pass runs).
- Cycles and self-references are compile-time errors.
- _subjectAsObject scoping is preserved through inlining.
- dependsOn is recomputed after resolution, so partial-graph requirements
  reach transitively through composed evidence.
- buildDirectRule/buildPredicateRule now apply subject-scoping to top-level
  PredicateCall evidence bodies (latent gap, previously missed).
- validation: reject relation names shared across facts/sources/evidence/
  measures (a collision silently overwrote configs and read as a false cycle).

Tests: EvidenceComposition (9), DSLRuntime transitive requiredFacts, oracle
campaign composition construct, illegal-mutations cycle + cross-kind cases.

tests/DSLRuntime.test.js

@@ -161,4 +161,31 @@ describe('DSLRuntime', () => {
     rt.addNode('doc:9', 'Doc', {});
     await assert.rejects(() => rt.check('u:1', 'does_not_exist', 'doc:9'), /unknown relation/);
   });
+
+  it('derives transitive required facts through evidence composition', async () => {
+    const dsl = `
+      definition Employee { id: string }
+      definition Doc { id: string }
+      fact *owns(user: Employee, doc: Doc)
+      fact *banned(user: Employee)
+      evidence can_read(user: Employee, doc: Doc) { owns(user, doc) }
+      evidence can_open(user: Employee, doc: Doc) { WHEN can_read(user, doc) UNLESS banned(user) }
+    `;
+    const rt = new DSLRuntime(new Arbiter()).compile(dsl, 'rt-comp');
+    rt.addNode('u:1', 'Employee', {});
+    rt.addNode('doc:9', 'Doc', {});
+    // can_open composes can_read, so its requirements reach through to owns.
+    assert.deepEqual(rt.requiredFacts('can_open'), ['owns', 'banned']);
+    const granted = await rt.check('u:1', 'can_open', 'doc:9', {
+      factProviders: { owns: async () => 0.9, banned: async () => 0 }
+    });
+    assert.equal(granted.possibility, 0.9);
+    assert.equal(granted.reason, 'allow_rule_matched');
+    assert.deepEqual(granted.providedFacts, ['owns', 'banned']);
+    const denied = await rt.check('u:1', 'can_open', 'doc:9', {
+      factProviders: { owns: async () => 0.9, banned: async () => 1 }
+    });
+    assert.equal(denied.possibility, 0);
+    assert.equal(denied.reason, 'defeated_by_unless');
+  });
 });

tests/EvidenceComposition.test.js

@@ -0,0 +1,154 @@
+/**
+ * tests/EvidenceComposition.test.js — referencing a derived evidence as a
+ * sub-rule of another evidence (WHEN can_read(user, doc) where can_read is
+ * itself an evidence).
+ *
+ * Composition is resolved at COMPILE time: the generator inlines each
+ * evidence reference with the referenced evidence's own config (a linker
+ * pass that handles forward references and rejects cycles), so the engine
+ * evaluates a fully-resolved, acyclic config tree.
+ */
+import { describe, it } from 'node:test';
+import assert from 'node:assert/strict';
+import { Arbiter } from '@arbiter/core';
+import { DSLCompiler } from '../src/DSLCompiler.js';
+
+const DEFS = `
+  definition Employee { id: string }
+  definition Group { id: string }
+  definition Doc { id: string }
+  fact owns(user: Employee, doc: Doc)
+  fact *trusted(user: Employee)
+  fact member_of(user: Employee, group: Group)
+  fact can_access(group: Group, doc: Doc)
+`;
+
+function compile(dsl, name = 'compose') {
+  const arb = new Arbiter();
+  const compiler = new DSLCompiler(arb);
+  const result = compiler.compile(dsl, name);
+  return { arb, result };
+}
+
+describe('Evidence composition', () => {
+  it('composes a direct evidence into another evidence', () => {
+    const { arb, result } = compile(`
+      ${DEFS}
+      evidence can_read(user: Employee, doc: Doc) { owns(user, doc) }
+      evidence can_browse(user: Employee, doc: Doc) { can_read(user, doc) }
+    `);
+    assert.ok(result.success, JSON.stringify(result.errors));
+    arb.addNode('u:1', 'Employee'); arb.addNode('doc:9', 'Doc');
+    arb.addRelation('u:1', 'owns', 'doc:9', { possibility: 0.8 });
+    const res = arb.check('u:1', 'can_browse', 'doc:9');
+    assert.equal(res.possibility, 0.8);
+    // The reference is inlined to the underlying fact config.
+    assert.equal(arb.relationConfigs.get('can_browse').type, 'direct');
+    assert.equal(arb.relationConfigs.get('can_browse').relation, 'owns');
+  });
+
+  it('composes an evidence inside a defeasible WHEN/UNLESS', () => {
+    const { arb, result } = compile(`
+      ${DEFS}
+      evidence can_read(user: Employee, doc: Doc) { owns(user, doc) }
+      evidence can_open(user: Employee, doc: Doc) { WHEN can_read(user, doc) UNLESS trusted(user) }
+    `);
+    assert.ok(result.success, JSON.stringify(result.errors));
+    arb.addNode('u:1', 'Employee'); arb.addNode('doc:9', 'Doc');
+    arb.addRelation('u:1', 'owns', 'doc:9', { possibility: 0.9 });
+    assert.equal(arb.check('u:1', 'can_open', 'doc:9').possibility, 0.9);
+    arb.addRelation('u:1', 'trusted', 'u:1', { possibility: 1.0 });
+    const denied = arb.check('u:1', 'can_open', 'doc:9');
+    assert.equal(denied.possibility, 0);
+    assert.equal(denied.reason, 'defeated_by_unless');
+  });
+
+  it('composes a chain evidence into another evidence', () => {
+    const { arb, result } = compile(`
+      ${DEFS}
+      evidence can_enter(user: Employee, doc: Doc) { member_of(user, *g) { can_access(g, doc) } }
+      evidence can_work(user: Employee, doc: Doc) { can_enter(user, doc) }
+    `);
+    assert.ok(result.success, JSON.stringify(result.errors));
+    arb.addNode('u:1', 'Employee'); arb.addNode('g:1', 'Group'); arb.addNode('doc:9', 'Doc');
+    arb.addRelation('u:1', 'member_of', 'g:1', { possibility: 1.0 });
+    arb.addRelation('g:1', 'can_access', 'doc:9', { possibility: 0.7 });
+    const res = arb.check('u:1', 'can_work', 'doc:9');
+    assert.equal(res.possibility, 0.7);
+    assert.equal(arb.relationConfigs.get('can_work').type, 'chain');
+  });
+
+  it('composes transitively (A → B → fact) and re-derives dependencies', () => {
+    const { arb, result } = compile(`
+      ${DEFS}
+      evidence can_read(user: Employee, doc: Doc) { owns(user, doc) }
+      evidence can_browse(user: Employee, doc: Doc) { can_read(user, doc) }
+      evidence can_open(user: Employee, doc: Doc) { can_browse(user, doc) }
+    `);
+    assert.ok(result.success, JSON.stringify(result.errors));
+    arb.addNode('u:1', 'Employee'); arb.addNode('doc:9', 'Doc');
+    arb.addRelation('u:1', 'owns', 'doc:9', { possibility: 0.6 });
+    assert.equal(arb.check('u:1', 'can_open', 'doc:9').possibility, 0.6);
+    assert.deepEqual(arb.relationConfigs.get('can_open').dependsOn, ['owns']);
+  });
+
+  it('composes a value-carrying evidence and preserves subject-as-object scope', () => {
+    const { arb, result } = compile(`
+      ${DEFS}
+      fact *user_risk(user: Employee, value: number)
+      evidence risk_ok(user: Employee, doc: Doc) { user_risk(user, 1) }
+      evidence can_proceed(user: Employee, doc: Doc) { risk_ok(user, doc) }
+    `);
+    assert.ok(result.success, JSON.stringify(result.errors));
+    arb.addNode('u:1', 'Employee'); arb.addNode('doc:9', 'Doc');
+    arb.addRelation('u:1', 'user_risk', 'u:1', { possibility: 1.0, value: 1 });
+    const res = arb.check('u:1', 'can_proceed', 'doc:9');
+    assert.equal(res.possibility, 1);
+  });
+
+  it('composes evidence inside a comparator operand', () => {
+    const { arb, result } = compile(`
+      ${DEFS}
+      fact *user_risk(user: Employee, value: number)
+      fact *risk_limit(doc: Doc, value: number)
+      evidence user_risk_ok(user: Employee, doc: Doc) { user_risk(user, 1) }
+      evidence can_proceed(user: Employee, doc: Doc) { user_risk_ok(user, doc) }
+    `);
+    assert.ok(result.success, JSON.stringify(result.errors));
+    arb.addNode('u:1', 'Employee'); arb.addNode('doc:9', 'Doc');
+    arb.addRelation('u:1', 'user_risk', 'u:1', { possibility: 1.0, value: 1 });
+    assert.equal(arb.check('u:1', 'can_proceed', 'doc:9').possibility, 1);
+  });
+
+  it('rejects cyclic evidence references at compile time', () => {
+    const { result } = compile(`
+      ${DEFS}
+      evidence a(user: Employee, doc: Doc) { b(user, doc) }
+      evidence b(user: Employee, doc: Doc) { a(user, doc) }
+    `);
+    assert.equal(result.success, false);
+    assert.ok(result.errors.some(e => /[Cc]yclic/.test(e)), JSON.stringify(result.errors));
+  });
+
+  it('rejects self-referencing evidence at compile time', () => {
+    const { result } = compile(`
+      ${DEFS}
+      evidence a(user: Employee, doc: Doc) { a(user, doc) }
+    `);
+    assert.equal(result.success, false);
+    assert.ok(result.errors.some(e => /[Cc]yclic/.test(e)), JSON.stringify(result.errors));
+  });
+
+  it('keeps the referenced evidence checkable in its own right', () => {
+    const { arb, result } = compile(`
+      ${DEFS}
+      evidence can_read(user: Employee, doc: Doc) { owns(user, doc) }
+      evidence can_browse(user: Employee, doc: Doc) { can_read(user, doc) }
+    `);
+    assert.ok(result.success);
+    arb.addNode('u:1', 'Employee'); arb.addNode('doc:9', 'Doc');
+    arb.addRelation('u:1', 'owns', 'doc:9', { possibility: 0.5 });
+    assert.equal(arb.check('u:1', 'can_read', 'doc:9').possibility, 0.5);
+    assert.equal(arb.check('u:1', 'can_browse', 'doc:9').possibility, 0.5);
+  });
+});

tests/rigor/dsl-generative-oracle.test.js

@@ -106,11 +106,28 @@ function buildProgram(kind, ps) {
       oracle = pG * pM;
       break;
     }
+    case 'composition': {
+      // can_via composes the direct evidence can_read, which reads the owns
+      // edge — an evidence-in-evidence reference resolved at compile time.
+      const [pOwn] = ps;
+      evidence = `evidence can_read(user: Employee, doc: Doc) { owns(user, doc) }
+                  evidence can_via(user: Employee, doc: Doc) { can_read(user, doc) }`;
+      edges.push({ src: 'u:1', relation: 'owns', dst: 'doc:9', possibility: pOwn });
+      oracle = pOwn;
+      break;
+    }
     default:
       throw new Error(`unknown construct: ${kind}`);
   }
 
-  return { dsl: FACTS + evidence, edges, oracle, relation: evidence.match(/evidence (\w+)/)[1] };
+  return {
+    dsl: FACTS + evidence,
+    edges,
+    oracle,
+    // Check the LAST evidence declaration: the composition construct declares
+    // two evidences (can_read + can_via), and the composed one is the target.
+    relation: [...evidence.matchAll(/evidence\s+(\w+)/g)].at(-1)[1]
+  };
 }
 
 function runCheck({ kind, ps }) {
@@ -134,7 +151,7 @@ function runCheck({ kind, ps }) {
 }
 
 const CONSTRUCTS = ['direct', 'chain', 'tuple_to_userset', 'fusion_min', 'fusion_max',
-  'when_unless', 'never_always', 'requires_when'];
+  'when_unless', 'never_always', 'requires_when', 'composition'];
 
 describe('DSL generative oracle parity (rigor)', () => {
   it('generated legal DSL compiles and every check matches the oracle', async () => {

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

@@ -81,6 +81,21 @@ const MUTATIONS = {
     desc: 'evidence declared with mismatched parameter arity',
     mustFail: true,
     apply: () => VALID_DSL.replace('evidence can_read(user: Employee, doc: Doc) { owns(user, doc) }', 'evidence can_read(user: Employee) { owns(user, doc) }')
+  },
+  cyclic_evidence_ref: {
+    desc: 'two evidences referencing each other (cycle)',
+    mustFail: true,
+    apply: () => VALID_DSL + `
+  evidence can_cyc_a(user: Employee, doc: Doc) { can_cyc_b(user, doc) }
+  evidence can_cyc_b(user: Employee, doc: Doc) { can_cyc_a(user, doc) }`
+  },
+  cross_kind_collision: {
+    desc: 'fact and evidence sharing a relation name',
+    mustFail: true,
+    apply: () => VALID_DSL.replace(
+      '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) }'
+    )
   }
 };