feat: Donut voiced-interview slice-3 — retire-ready KB-recall bridge

Grounds the interview character in the ingested corpus while she stays in-voice.
Tier-3 agents are tool-less by design in v1, so this is the consumer-side
workaround (DEC-6, worldtree-dev ruling): per opted-in interview turn, ratatoskr
consults Mimir out-of-band, extracts the passages, and pins them as memory_context
on the character's turn. She frames the pinned corpus as her own memory.

- src/ratatoskr/kb_bridge.py (new, RETIRE-READY): pin_kb_context — THE single seam
  (INV-KB-1). Allowlist-gated (INV-KB-4: ratatoskr:donut only), hard-timeout-bounded,
  degrades to [] on any failure/timeout/empty (INV-KB-3, never raises; CancelledError
  propagates). Imports nothing from the SDK-adapter / TTS core. aclosing() closes the
  SDK stream deterministically on the DoneEvent break.
- wt.stream_turn: memory_context passthrough (defaults None — inert for every other
  caller and for the bridge's own retirement). Seam-review catch: the contract's
  original touch list undercounted wt.py by one file (recorded in the contract).
- web/server.py: TurnHandle.agent_id + the single pin_kb_context call-site on the
  turn path; the browser now sends agent_id so the allowlist can gate.
- web/static/index.html: the turn POST carries agent_id.

Consult prompt tuned live: "search_library EXACTLY ONCE, no read_note" converges
Mimir in ~3-15s (the softer "do one search" phrasing looped past 25s on
conversational questions).

TDD: 12 kb_bridge unit tests + wt memory_context forwarding + 2 server wiring tests
(531 green). Live-smoked on :8081/b128: pin_kb_context grounds in the DCC corpus
(real excerpts, <20s) and Donut answers in-voice; degrades cleanly on a slow consult.
KNOWN LIMIT surfaced (not a bridge defect): DCC's fiction index is weak (failed
backfill, a worldtree-dev item), so grounding is opportunistic — the bridge's real
payoff is a corpus the model does not already know.

Per docs/contracts/donut_voiced_interview.contract.md (slice 3 of 3).

tests/test_wt.py

@@ -360,7 +360,15 @@ class TestStreamTurn:
         fake = _FakeSessions(events=[e1, e2])
         got = await _drain(stream_turn(_wt(fake), "s-1", "hello"))
         assert got == [e1, e2]
-        assert fake.calls[-1] == ("stream_turn", ("s-1", "hello"), {})
+        # memory_context defaults to None (forwarded verbatim; inert for every caller
+        # that doesn't pin corpus context via the KB-recall bridge).
+        assert fake.calls[-1] == ("stream_turn", ("s-1", "hello"), {"memory_context": None})
+
+    async def test_forwards_memory_context_verbatim(self) -> None:
+        mc = [{"kind": "corpus_reference", "text": "the pinned passage"}]
+        fake = _FakeSessions(events=[])
+        await _drain(stream_turn(_wt(fake), "s-1", "hi", memory_context=mc))
+        assert fake.calls[-1] == ("stream_turn", ("s-1", "hi"), {"memory_context": mc})
 
     async def test_session_retired_maps_to_session_api_failed(self) -> None:
         fake = _FakeSessions(