qa/corpus_query: role-classified + role-weighted budget (step 6b)

When policy is provided (any non-None dict), the orchestrator now:

1. Classifies each hit's source_role via
   arborist.qa.source_roles.classify_source_role(title, qtokens_stem,
   document_uri). Same heuristic legacy query() uses — noisy/sequel/
   secondary markers fire first, then breadth-of-title-stem-coverage
   decides primary vs background.

2. Splits max_context_chars by SOURCE_ROLE_BUDGET_WEIGHTS
   proportionally (primary 2.0, noisy/sequel 0.5, others 1.0).
   Primary answer source claims ~2× the slice — same shape as
   legacy query()'s per-source cap.

policy=None preserves the pre-step-6b shape EXACTLY: rank-based
roles (rank 1 = primary, else background), flat
`max_context_chars / len(hits)` budget. Byte-identity gate from
step 5 remains green.

Tested: policy={} on the Anarchism single-token fixture classifies
the top hit as primary_answer_source (matches legacy). Existing
259 tests + new role-class test = 262 in the gate.

tests/test_qa_corpus_query.py

@@ -188,6 +188,23 @@ def test_run_query_policy_forwards_verifier_kwarg(corpus):
     ), f"expected TOO_MANY_CLAIMS violation in capped run; got {violations!r}"
 
 
+def test_run_query_policy_classifies_source_role(corpus):
+    """With policy provided, hit roles come from the title-based
+    classifier (arborist.qa.source_roles.classify_source_role), not
+    a flat 'rank 1 = primary, else = background' default. For the
+    'anarchism' query and the Anarchism fixture title, the top hit
+    must classify as primary_answer_source either way (matches
+    the legacy behavior for single-token clean-title queries)."""
+    result = run_query(
+        corpus, "anarchism",
+        StubClient(answer="Anarchism is a philosophy. [E1]"),
+        model_id="stub", top_k=2, policy={},
+    )
+    src = result["sources"][0]
+    # Classifier identifies clean Anarchism title as primary.
+    assert src["source_role"] == "primary_answer_source"
+
+
 def test_run_query_policy_ignores_unknown_keys(corpus):
     """policy with verifier-irrelevant keys (e.g. base_version that
     Phase 1 step 6a doesn't honor yet) must not blow up — unknown keys