fix: witness role parametric in prompt assembly (T71.1)

Phase 2 T46 pinned the witness mask contract on search_memories with a
witness_role parameter (host/guest/you). The prompt-assembly call site
in assemble_narrative_prompt was hardcoded to "host", which silently
returned the wrong rows when the speaker was the guest bot.

Derive the witness role from chat membership via a new private helper
_witness_role_for(speaker_bot_id, host_bot_id), and apply it at the
search_memories call. Behaviour is identical when the speaker is the
host (or when no guest is present); the fix is load-bearing only when
the guest bot is the speaker — exactly the scenario Phase 2 T43 added
support for.

Tests: pin both directions (host-as-speaker and guest-as-speaker) by
patching the imported search_memories reference and asserting the
witness_role argument the call site emits.

tests/test_prompt.py

@@ -452,6 +452,68 @@ def test_assemble_when_speaker_is_guest_orients_edges_correctly(tmp_path):
     assert "68/100" in body
 
 
+def test_speaker_is_guest_uses_guest_witness_role(tmp_path, monkeypatch):
+    """T71.1: when the guest is the speaker, ``search_memories`` is
+    called with ``witness_role="guest"``, not the previously-hardcoded
+    ``"host"``. Pins the parametric witness role at the prompt call site
+    so guest-as-speaker honours the witness mask via Phase 2 T46.
+    """
+    db = tmp_path / "t.db"
+    apply_migrations(db)
+    captured: dict = {}
+
+    def _fake_search(conn, owner_id, witness_role, query, k=4):
+        captured["owner_id"] = owner_id
+        captured["witness_role"] = witness_role
+        captured["query"] = query
+        return []
+
+    # Patch the imported reference inside the prompt module so the
+    # production call site uses the fake.
+    import chat.services.prompt as prompt_mod
+    monkeypatch.setattr(prompt_mod, "search_memories", _fake_search)
+
+    with open_db(db) as conn:
+        _seed_with_guest(conn)
+        # Guest as speaker — must request memories with witness_role="guest".
+        assemble_narrative_prompt(
+            conn,
+            chat_id="chat_bot_a",
+            speaker_bot_id="bot_b",
+            recent_dialogue=[],
+            # retrieved_memory_summaries=None forces the search path.
+            retrieved_memory_summaries=None,
+        )
+    assert captured["owner_id"] == "bot_b"
+    assert captured["witness_role"] == "guest"
+
+
+def test_speaker_is_host_uses_host_witness_role(tmp_path, monkeypatch):
+    """T71.1 (regression): host-as-speaker still queries with
+    ``witness_role="host"``."""
+    db = tmp_path / "t.db"
+    apply_migrations(db)
+    captured: dict = {}
+
+    def _fake_search(conn, owner_id, witness_role, query, k=4):
+        captured["witness_role"] = witness_role
+        return []
+
+    import chat.services.prompt as prompt_mod
+    monkeypatch.setattr(prompt_mod, "search_memories", _fake_search)
+
+    with open_db(db) as conn:
+        _seed_with_guest(conn)
+        assemble_narrative_prompt(
+            conn,
+            chat_id="chat_bot_a",
+            speaker_bot_id="bot_a",  # host as speaker
+            recent_dialogue=[],
+            retrieved_memory_summaries=None,
+        )
+    assert captured["witness_role"] == "host"
+
+
 def test_assemble_with_tight_budget_drops_guest_activity_first(tmp_path):
     """Under tight budget MUST blocks survive but SHOULD-tier guest
     activity is dropped first."""