perf(MAT): cut bot reply latency — stream, skip redundant rewrite, non-blocking persist

Latency was dominated by the LLM call chain, not the 10-message context window.
Three fixes land together in the chat pipeline in bot.py:

1. Stream the main LLM call (new _stream_chat_completion helper) and
   progressively edit the Matrix message via m.replace. Suppress visible
   streaming during tool-calling iterations so the user never sees rolled-back
   text. Final send is an authoritative edit that guarantees the full reply.

2. Gate _rewrite_query behind a pronoun/deictic heuristic (EN/DE/FR). When a
   message has no references needing resolution we skip the extra Haiku
   round-trip entirely and feed the original message to RAG directly.

3. Fire-and-forget the post-reply memory + chunk persistence with asyncio
   background tasks so a slow extraction no longer blocks the next inbound
   message. 20s timeout preserved inside the bg task; exceptions logged.

Added unit test for the pronoun heuristic (EN/DE/FR positive + negative cases,
short/empty messages).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

tests/test_needs_query_rewrite.py

@@ -0,0 +1,41 @@
+"""Heuristic gate for `_rewrite_query` (bot.py). Skips the LLM round-trip when
+the message has no pronouns or deictic references that would need context."""
+
+from bot import Bot
+
+
+def _needs(msg: str) -> bool:
+    return Bot._needs_query_rewrite(msg)
+
+
+def test_short_message_skipped():
+    assert _needs("hi") is False
+    assert _needs("ok") is False
+
+
+def test_self_contained_no_pronouns_skipped():
+    assert _needs("What is the capital of France?") is False
+    assert _needs("Summarize the Q3 earnings report") is False
+    assert _needs("Wie ist das Wetter in Berlin morgen") is False
+
+
+def test_english_pronouns_trigger():
+    assert _needs("What does it mean?") is True
+    assert _needs("Can you fix that?") is True
+    assert _needs("Tell me more about them") is True
+
+
+def test_german_pronouns_trigger():
+    assert _needs("Was bedeutet das?") is True
+    assert _needs("Kannst du es noch einmal erklären") is True
+    assert _needs("Wer sind sie?") is True
+
+
+def test_french_pronouns_trigger():
+    assert _needs("Qu'est-ce que ça veut dire?") is True
+    assert _needs("Parle-moi de lui") is True
+
+
+def test_empty_or_whitespace():
+    assert _needs("") is False
+    assert _needs("   ") is False