Prevent same-second outgoing collision now that we can send faster.

Also add pending ack tracking

app/services/dm_ack_tracker.py

@@ -6,12 +6,26 @@ import time
 logger = logging.getLogger(__name__)
 
 PendingAck = tuple[int, float, int]
+BUFFERED_ACK_TTL_SECONDS = 30.0
 
 _pending_acks: dict[str, PendingAck] = {}
+_buffered_acks: dict[str, float] = {}
 
 
-def track_pending_ack(expected_ack: str, message_id: int, timeout_ms: int) -> None:
-    """Track an expected ACK code for an outgoing direct message."""
+def track_pending_ack(expected_ack: str, message_id: int, timeout_ms: int) -> bool:
+    """Track an expected ACK code for an outgoing direct message.
+
+    Returns True when the ACK was already observed and buffered before registration.
+    """
+    buffered_at = _buffered_acks.pop(expected_ack, None)
+    if buffered_at is not None:
+        logger.debug(
+            "Matched buffered ACK %s immediately for message %d",
+            expected_ack,
+            message_id,
+        )
+        return True
+
     _pending_acks[expected_ack] = (message_id, time.time(), timeout_ms)
     logger.debug(
         "Tracking pending ACK %s for message %d (timeout %dms)",
@@ -19,6 +33,13 @@ def track_pending_ack(expected_ack: str, message_id: int, timeout_ms: int) -> No
         message_id,
         timeout_ms,
     )
+    return False
+
+
+def buffer_unmatched_ack(ack_code: str) -> None:
+    """Remember an ACK that arrived before its message registration."""
+    _buffered_acks[ack_code] = time.time()
+    logger.debug("Buffered unmatched ACK %s for late registration", ack_code)
 
 
 def cleanup_expired_acks() -> None:
@@ -33,6 +54,15 @@ def cleanup_expired_acks() -> None:
         del _pending_acks[code]
         logger.debug("Expired pending ACK %s", code)
 
+    expired_buffered_codes = [
+        code
+        for code, buffered_at in _buffered_acks.items()
+        if now - buffered_at > BUFFERED_ACK_TTL_SECONDS
+    ]
+    for code in expired_buffered_codes:
+        del _buffered_acks[code]
+        logger.debug("Expired buffered ACK %s", code)
+
 
 def pop_pending_ack(ack_code: str) -> int | None:
     """Claim the tracked message ID for an ACK code if present."""