Fix ADR-0025: IPCQ direction addressing via address-based matching

2-rank bidirectional ring deadlock: when E and W neighbors point to the
same peer, sender-coord matching in _handle_meta_arrival / _credit_worker
picked the first direction in dict order, landing data in the wrong rx
slot relative to what the kernel recv(W) was waiting on.

Fix (ADR-0025 D1/D2/D3):
- install.reverse_direction: prefer OPPOSITE direction (E↔W, N↔S) when
  peer has it pointing back to us; fallback to any matching for
  topologies without opposite convention (tree_binary parent/child).
- _handle_meta_arrival: match by token.dst_addr range against each qp's
  my_rx_base_pa + n_slots × slot_size window (unambiguous).
- _credit_worker: match by credit.dst_rx_base_pa == qp.peer.rx_base_pa.
- IpcqCreditMetadata: new dst_rx_base_pa field carrying receiver-side
  rx base; _delayed_credit_send fills it from the consuming qp.

Tests (Phase 1 → Phase 2):
- test_reverse_direction_opposite_preference_2rank_ring
- test_reverse_direction_opposite_preference_4rank_ring_sanity
- test_meta_arrival_matches_by_dst_addr_same_peer
- test_credit_matches_by_dst_rx_base_pa_same_peer
- Existing credit-return test updated with dst_rx_base_pa.

508 tests pass.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/kernbench/ccl/install.py

@@ -219,9 +219,24 @@ def install_ipcq(
         "neighbor_table": neighbor_table,
     }
 
-    def reverse_direction(my_rank: int, peer_rank: int) -> str | None:
-        """Find which direction in peer's neighbor table points back to my_rank."""
-        for d, target in neighbor_table[peer_rank].items():
+    _OPPOSITE_DIR = {"E": "W", "W": "E", "N": "S", "S": "N"}
+
+    def reverse_direction(my_rank: int, peer_rank: int, my_dir: str) -> str | None:
+        """Find peer's direction that reciprocates my_dir→peer_rank.
+
+        Prefer the OPPOSITE direction (E↔W, N↔S) when the peer has it
+        pointing back to us (ADR-0025 D1). This matters in 2-rank
+        bidirectional rings where both E and W on one side point to the
+        same peer — without the preference, dict-order first-match would
+        route data into the wrong rx slot. Falls back to any direction
+        pointing back for topologies without an opposite convention
+        (e.g. tree_binary's parent/child).
+        """
+        nt = neighbor_table[peer_rank]
+        opp = _OPPOSITE_DIR.get(my_dir)
+        if opp is not None and nt.get(opp) == my_rank:
+            return opp
+        for d, target in nt.items():
             if target == my_rank:
                 return d
         return None
@@ -234,7 +249,7 @@ def install_ipcq(
             if peer_rank is None:
                 continue
             peer_s, peer_c, peer_p = rank_pe[peer_rank]
-            peer_dir = reverse_direction(r, peer_rank)
+            peer_dir = reverse_direction(r, peer_rank, d)
             if peer_dir is None:
                 # Peer doesn't have a reverse entry — skip (asymmetric topology)
                 continue