ticket #000028 §2.6 sketch: refresh stale TODO post-#000027

Broader stale-map sweep across docs/, bench/, scripts/, Makefile, and
top-level surfaced one hit: the witness-mode example sketch in §2.6
of ticket #000028 still showed a no-op cache_lookup closure with
"TODO: wire post-#000027" — even though the real implementation in
arborist/qa/query.py:2158-2172 wired the cache leg in commit e19aed8
(2026-05-09), the same commit that closed both #000027 and #000028.

Updated the sketch to match the actual implementation: the cache
lookup closes over the prior persisted row's answer bytes (encoded
with surrogatepass for UTF safety), and the comparison is non-
tautological because cached_row is the row picked up BEFORE we'd
write a new one in this same call.

Same drift pattern as the #000028 §8 follow-up refresh (6d20aeb)
and the #000010/§000021/#000040 sweep (e84f453) and the repair.py
TODOs (8980e64): pre-implementation design notes don't get refreshed
after the implementation lands. Each instance is one session-waste
saved.

After this, both surfaces are clean:
- arborist/ + tests/ stale TODOs: zero (8980e64)
- docs/ + bench/ + scripts/ + Makefile + top-level: zero (this commit)

The remaining matches are all genuine future-work markers (raw_html
cache in async_web_fetcher.py:2309) or false-positives (\\uXXXX
escape-pattern docs in mesh/wire.py and tests/test_cli_render.py).

docs/tickets/ticket-000028-multi-modality-witness.md

@@ -261,10 +261,18 @@ if canonical_match is not None:
     witness = None
     if policy.get("canonical_witness_enabled", False):
         from arborist.qa.witness import run_witness
-        # Cache lookup is a no-op closure pre-#000027 (always returns
-        # None). Post-#000027 it's the actual canonical_cache lookup.
-        def _cache_lookup() -> bytes | None:
-            return None  # TODO: wire post-#000027
+        # Cache lookup closes over the prior persisted row's answer
+        # bytes (post-#000027 / e19aed8). The real implementation in
+        # arborist/qa/query.py:2158-2172 reads the cached_row picked
+        # up at the top of the canonical branch — comparing against
+        # that prior row, not anything we'd write in the same call,
+        # so the leg is non-tautological.
+        _cached_bytes = (
+            cached_row["answer_text"].encode("utf-8", errors="surrogatepass")
+            if cached_row is not None
+            else None
+        )
+        _cache_lookup = lambda: _cached_bytes  # noqa: E731
         witness = run_witness(
             question=question,
             pi_star_ref=pi_star_ref,