feat(retrieval): accent-fold (+30pp recall@1) + fold-search factory hardening

Second MEASURED fold-search win, and the instrument correcting my own
premature call. accent-fold ON vs OFF on the mined accent fixture:
recall@1 55% -> 85% (+30pp), rank-1 22/40 -> 34/40. recall@8 was
flat (95->98) — a too-lenient k nearly got a real lever wrongly
reverted; @1/@3 is the resolution that drives primary-source
selection. _accent_fold_variants: ASCII-fold then re-tokenise so a
diacritic title ("Béla Bartók", which _TITLE_TOKEN_RE otherwise
fragments to junk) matches the ASCII form a user types. Additive+
symmetric, no-op on pure-ASCII (zero effect on non-accent
queries/titles), mirrors _hyphen_fold_variants (#000007).

Also fixes a defect I shipped in a3ac653: an orphaned duplicate
body left as dead code after `return base` in _title_query_tokens
(unreachable — numeral-fold behaviour/measurement were valid — but
cruft; removed).

Fold-search factory, fanned out across the full survey backlog
(deterministic recall, no LLM, parallel — serial-by-caution was
halting in disguise):
- recall_at_k.py: returns rank -> recall@1/@3/@k from one retrieval
  (verified offline). A coarse k hides rank-only lifts.
- mine_questions.py: numeral/accent/hyphen/honorific/amp/brit
  ground-truth classes; fixtures committed.
- Measured @1 headroom verdicts: accent SHIP (this commit);
  honorific 45% / brit 50% = real headroom (build next); hyphen
  90% = existing #000007 already delivers, NOTHING to build (the
  measure-the-unmeasured-thing check pays off); amp 82% = no fold
  needed (prevalence-overranked, instrument kills it cheaply).

CLAUDE.md bench-maxing: two measured lessons codified — report
recall@1/@3/@k (a lenient k hides rank lifts; prevalence != miss-
rate), and fan out independent measurements (serial-by-caution is
halting). Full suite 2488 passed, 0 regressions (accent-fold is
hot-path in _title_query_tokens); real-path test (FakeSource->
ingest->query()->real _Hit).

tests/test_accent_fold.py

@@ -0,0 +1,83 @@
+"""Accent-fold: ASCII-folded variants for diacritic text, retrieval-side.
+
+Measured 2026-05-18 on the mined ground-truth fixture: recall@1
+55% -> 85% (+30pp), rank-1 22/40 -> 34/40. recall@8 was a near-
+miss-revert artifact (95->98, noise) — a too-lenient k hid a
+rank-only lift; @1/@3 is the resolution that drives primary-source
+selection. Same additive+symmetric discipline as
+`_hyphen_fold_variants` (#000007) / `_numeral_fold_variants`.
+
+Tested through the REAL path (FakeSource -> ingest -> query() ->
+real `_Hit`), not a hand-built object — the discipline the reverted
+disambiguation v1 violated.
+"""
+from __future__ import annotations
+
+from collections.abc import Iterator
+
+from arborist.qa.query import _accent_fold_variants, _title_query_tokens
+
+
+def test_variant_is_additive_symmetric_and_noop_on_ascii():
+    assert _accent_fold_variants("Béla Bartók") == {"bela", "bartok"}
+    assert _accent_fold_variants("André-Marie Ampère") >= {"andre", "ampere"}
+    # pure ASCII -> folds to itself -> empty -> zero effect (the
+    # additive-safety invariant that keeps non-accent queries intact)
+    assert _accent_fold_variants("what is the capital of france?") == set()
+    assert _accent_fold_variants("who painted the mona lisa?") == set()
+
+
+def test_title_query_tokens_bridges_ascii_query_to_accented_title():
+    q = _title_query_tokens("what is Bela Bartok?")
+    t = _title_query_tokens("Béla Bartók")
+    # the accented title otherwise fragments ("Béla"->"B","la") and
+    # never overlaps the ASCII form; the fold restores the bridge.
+    assert {"bela", "bartok"} <= (q & t)
+    # plain ASCII query unaffected
+    assert _title_query_tokens("who painted the mona lisa?") == {
+        "painted", "mona", "lisa"
+    }
+
+
+def test_query_retrieves_accented_title_for_ascii_question(tmp_path):
+    from arborist.qa.client import StubClient
+    from arborist.document import Document
+    from arborist.ingest import ingest_source
+    from arborist.qa.query import DEFAULT_QUERY_POLICY, query
+    from arborist.source import Source
+    from arborist.store import connect
+
+    class FakeSource(Source):
+        source_type = "test"
+
+        def __init__(self, ds):
+            self.ds = ds
+
+        def iter_documents(self) -> Iterator[Document]:
+            yield from self.ds
+
+    shard = tmp_path / "shard.db"
+    c = connect(shard)
+    try:
+        ingest_source(c, FakeSource([
+            Document(uri="t://1",
+                     content="Béla Bartók was a Hungarian composer. "
+                             "Béla Bartók pioneered ethnomusicology. " * 12,
+                     source_type="test", title="Béla Bartók"),
+            Document(uri="t://2",
+                     content="Unrelated article about gardening. " * 12,
+                     source_type="test", title="Gardening"),
+        ]))
+    finally:
+        c.close()
+    r = query(
+        question="what is Bela Bartok?",
+        qa_db=tmp_path / "qa.db",
+        chat_client=StubClient(answer="Béla Bartók was a composer. [E1]\n"),
+        model_id="stub",
+        single_db=shard,
+        policy=dict(DEFAULT_QUERY_POLICY, answer_mode="claim_lattice_pointer"),
+    )
+    titles = {(s.get("title") or "") for s in (r.get("sources") or [])}
+    assert r["status"] != "error"
+    assert "Béla Bartók" in titles      # accented title surfaced from ASCII q