feat(chat): client turn-id makes fire-and-forget bulletproof

Brian fires a quick message then locks his phone to go play the hand — which drops
the SSE stream, and on wake the UI re-fires via the blocking fallback. The prior
(session, message) + 20s window caught the near-simultaneous case but not a re-fire
minutes later.

Now the UI stamps each send with a unique turnId (crypto.randomUUID) and carries the
SAME id on both the stream and the fallback; the server dedupes on it. Bulletproof
regardless of how long he's away — lock for an hour, come back, still exactly one
execution and one log — and a genuinely new send gets a fresh id so nothing legit is
swallowed. Id-keyed turns keep a long (1h) window; requests without an id keep the
short (session, msg) window for near-simultaneous dupes.

Verified end-to-end: two POSTs with the same turnId → one reply, one persisted
exchange pair (the duplicate reused the owner's result). 9 dedup tests; suite 235.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/test_turn_dedup.py

@@ -64,6 +64,30 @@ def test_fresh_message_after_window_runs_again():
     # a completed turn lingers only briefly; simulate expiry and confirm re-ownership
     o1, rec = chat._claim_turn("s1", "later resend")
     chat._finish_turn(rec, "first")
-    rec["ts"] -= chat._TURN_TTL + 1          # age it past the window
+    rec["ts"] -= chat._TURN_TTL_MSG + 1       # age it past the (session,msg) window
     o2, _ = chat._claim_turn("s1", "later resend")
     assert o1 and o2                          # a genuine later resend runs fresh
+
+
+# --- client turn-id keying (the fire-and-forget guarantee) ----------------
+
+def test_same_turn_id_dedupes_regardless_of_message():
+    # the fallback may resend the SAME id; dedupe on the id, not the text
+    o1, r1 = chat._claim_turn("s1", "a hand", turn_id="tid-123")
+    o2, r2 = chat._claim_turn("s1", "a hand", turn_id="tid-123")
+    assert o1 is True and o2 is False and r1 is r2
+
+
+def test_different_turn_ids_each_own():
+    o1, _ = chat._claim_turn("s1", "same text", turn_id="tid-1")
+    o2, _ = chat._claim_turn("s1", "same text", turn_id="tid-2")
+    assert o1 and o2                          # a genuinely new send never gets swallowed
+
+
+def test_turn_id_window_survives_long_after_the_msg_window():
+    # locked-phone case: the re-fire can arrive minutes later and must still dedupe
+    o1, rec = chat._claim_turn("s1", "big hand", turn_id="tid-lock")
+    chat._finish_turn(rec, "cached")
+    rec["ts"] -= chat._TURN_TTL_MSG + 60      # well past the short window, but not the id window
+    o2, r2 = chat._claim_turn("s1", "big hand", turn_id="tid-lock")
+    assert o1 and o2 is False and r2["reply"] == "cached"