qa(retrieval): fix 50→100s hangs in title-LIKE & FTS5 OR-fallback

Both retrieval phases were doing O(corpus × tokens) full-scan work
on long queries — synonym work surfaced the latent issues but they
predate it. Hermes is 6-8s; retrieval needs to be sub-second.

(1) `_search_titles` (LIKE-based title backup search) is fundamentally
    O(corpus × |accept_tokens|) — `LOWER(title) LIKE '%tok%'` can't
    use any index. 50 patterns × 870k docs × 4 shards = ~56s pure
    full-scan. Skipped for queries with > 5 accept_tokens, where
    FTS5 BM25 already returns better candidates. Title-LIKE remains
    a backup for short focused queries (1-5 tokens).

(2) FTS5 OR-mode fallback was OR-joining ALL post-stopword query
    tokens. A 19-token query OR-clause matched millions of docs;
    BM25 then ranked them all. 13s/shard observed.
    Capped to top-5 LONGEST tokens (proxy for rarity / topical
    specificity). Long words like "neurotechnology" matter; short
    words like "soon" don't. OR-mode now 0.25s/shard.

(3) Stopword set extended with 6 connector words (one, some,
    another, without, soon, currently). Conservative addition —
    these have zero topical signal. Reduces the AND-mode token
    count, raising the chance AND succeeds before falling back to
    OR.

Verified live: the 19-token brain-tech query that hung indefinitely
now returns in 18s (Hermes + verify + render dominate). Source
`_search_corpus` no longer expands synonyms for the title-LIKE call
either — synonym expansion stays in `_filter_by_title_relevance`
(post-retrieval, in-memory, cheap regardless of accept-set size)
where it actually does useful work.

Tests: 641 passed (no regression).

aborist/qa/query.py

@@ -911,9 +911,17 @@ def _search_corpus(
     out-ranks FTS5 body hits so the actual topic article rises to the top.
     """
     qtokens = _title_query_tokens(question)
-    # Synonym expansion: a query for "athlon" also fetches AMD-titled docs.
-    # Reads concept_relations cross-shard via the shards_dir already in scope.
-    accept_tokens = synonym_expand(qtokens, shards_dir=shards_dir)
+    # Title-LIKE backup search uses ORIGINAL qtokens only — NOT synonym-
+    # expanded. With expansion, this path becomes O(corpus × |accept|)
+    # full-scan because LOWER(title) LIKE '%tok%' can't use any index;
+    # 50 LIKE patterns × 870k docs × 4 shards = ~56s of pure waste.
+    # Synonym expansion stays in `_filter_by_title_relevance` (post-
+    # retrieval, in-memory, cheap regardless of accept-set size) where
+    # it actually does useful work surfacing synonym-related titles
+    # from the FTS5 BM25 hits. Title-LIKE is a thin backup for docs
+    # whose body is short on query terms but whose title is the literal
+    # topic — that case is well-served by the original qtokens.
+    accept_tokens = set(qtokens)
     paths: list[Path]
     if shards_dir is not None:
         paths = discover_shards(shards_dir)
@@ -980,7 +988,20 @@ def _search_corpus(
             accept_stems = {
                 _stem_token_for_match(t) for t in accept_tokens
             }
-            for r in _search_titles(conn, list(accept_tokens), over_fetch):
+            # Title-LIKE is O(corpus × |tokens|) full-scan (LIKE '%tok%'
+            # can't use any index). Skip it for long queries (>5
+            # tokens) where FTS5 BM25 already returns better candidates
+            # than title-LIKE could. Title-LIKE remains a backup for
+            # short focused queries (1-5 tokens) where FTS5 might miss
+            # the literal-topic article in favor of body-frequency
+            # noise. The 5-token threshold matches the practical limit
+            # before title-LIKE blows the per-shard budget at ~1s/token.
+            title_search_rows = (
+                _search_titles(conn, list(accept_tokens), over_fetch)
+                if len(accept_tokens) <= 5
+                else []
+            )
+            for r in title_search_rows:
                 title_norm = (r["title"] or "").replace("_", " ")
                 title_stems = {
                     _stem_token_for_match(t)

aborist/search/fts5.py

@@ -27,10 +27,22 @@ _FTS5_STOPWORDS = frozenset(
     tell show describe explain summarize say give list find make
     please
     all there know everything anything something
+
+    one some another without soon currently
     """.split()
 )
 
 
+# OR-mode fallback hits SQLite FTS5 with one clause per token. With 15+
+# tokens including common English connectors it matches millions of
+# docs, then BM25 has to rank them all to pick the top-K — 13s/shard
+# observed on a 3.47M-doc corpus. Long queries that fail AND-mode
+# should not pay for an OR-mode that's effectively a full-corpus scan.
+# Cap OR-mode at the top-N LONGEST tokens (proxy for rarity / topical
+# specificity — "neurotechnology" matters; "soon" doesn't).
+_OR_FALLBACK_MAX_TOKENS = 5
+
+
 def _query_tokens(query: str) -> list[str]:
     raw = _FTS5_TOKEN_RE.findall(query)
     return [t for t in raw if t.lower() not in _FTS5_STOPWORDS and len(t) > 1]
@@ -48,8 +60,12 @@ def _escape_fts5(query: str, *, mode: str = "and") -> str:
 
     - mode='and' (default): every content token must appear in the doc.
       Strict relevance — keeps unrelated docs out of the context window.
-    - mode='or':  any token matches. Used as a fallback when AND returns
-      zero hits.
+    - mode='or':  the top-N LONGEST tokens (proxy for topical specificity)
+      get OR-joined. Used as a fallback when AND returns zero hits.
+      Capped at ``_OR_FALLBACK_MAX_TOKENS`` so OR-mode doesn't degrade
+      into a full-corpus scan on long noisy queries (a 19-token OR
+      clause matches millions of docs and forces BM25 to rank them all
+      — 13s/shard observed pre-cap).
 
     All-stopword queries fall back to OR over the raw tokens so they
     still find something instead of crashing FTS5 with an empty MATCH.
@@ -59,8 +75,16 @@ def _escape_fts5(query: str, *, mode: str = "and") -> str:
         raw = _FTS5_TOKEN_RE.findall(query)
         tokens = [t for t in raw if len(t) > 1] or ['""']
         sep = " OR "
+    elif mode == "and":
+        sep = " AND "
     else:
-        sep = " AND " if mode == "and" else " OR "
+        # OR fallback: keep only the rarest tokens (approximated by
+        # longest — long words tend to be more topical / rarer in the
+        # corpus). Bounds the per-clause cost so OR-mode terminates
+        # quickly instead of scanning the corpus.
+        sep = " OR "
+        if len(tokens) > _OR_FALLBACK_MAX_TOKENS:
+            tokens = sorted(tokens, key=len, reverse=True)[:_OR_FALLBACK_MAX_TOKENS]
     return sep.join(_quote(t) for t in tokens)