ADR-0023 D9: blocking credit-emit with full-path latency

PE_IPCQ._handle_recv now yields-from _delayed_credit_send instead of
spawning it as a fork, so the receiver's pe_exec_ns includes the
credit-return cost. _credit_latency_ns switches from
compute_drain_ns(path, 16) to compute_path_latency_ns(path, 16) and
fixes a latent find_path bug where the destination lacked the
".pe_dma" suffix (silently returned 0 ns under the bare except).

Net effect on h3/h4 inter-cube pe-to-pe latency: IPCQ >= raw DMA at
every size, matching real-HW posted-write semantics. tl.send remains
fire-and-forget. ADR-0023 D9 amended; new diagnostic test
tests/test_pe_to_pe_diagnostic.py captures per-PE pe_exec_ns, paths,
drain, and meta-arrival timing.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/kernbench/components/builtin/pe_ipcq.py

@@ -338,9 +338,13 @@ class PeIpcqComponent(ComponentBase):
                 nbytes=req.result_data.get("nbytes", 0),
             )
 
-        # Fast path credit return — bottleneck BW based latency
-        env.process(
-            self._delayed_credit_send(env, direction, qp["peer_credit_store"], qp["my_tail"])
+        # Credit return: recv blocks on credit-emit so the protocol cost
+        # (full path latency to deliver the credit metadata back to the
+        # sender) is reflected in the recv's pe_exec_ns. Models the IPCQ
+        # control-plane completing the consume-acknowledgement before
+        # recv returns to the kernel.
+        yield from self._delayed_credit_send(
+            env, direction, qp["peer_credit_store"], qp["my_tail"],
         )
 
         if not req.done.triggered:
@@ -455,7 +459,12 @@ class PeIpcqComponent(ComponentBase):
         yield peer_credit_store.put(meta)
 
     def _credit_latency_ns(self, direction: str) -> float:
-        """Compute credit fast path latency = credit_size / bottleneck_bw.
+        """Full path latency for the credit-return packet.
+
+        Pays per-node overhead + edge prop + drain along the same fabric
+        the data took. PathRouter.find_path() auto-appends ".pe_dma" to
+        the source only, so the destination MUST be spelled with the
+        explicit ".pe_dma" suffix.
 
         Falls back to 0 when ctx/router is unavailable (unit-test mode).
         """
@@ -463,10 +472,12 @@ class PeIpcqComponent(ComponentBase):
             return 0.0
         qp = self._queue_pairs[direction]
         peer = qp["peer"]
-        peer_pe_prefix = f"sip{peer.sip}.cube{peer.cube}.pe{peer.pe}"
+        peer_pe_dma = f"sip{peer.sip}.cube{peer.cube}.pe{peer.pe}.pe_dma"
         try:
-            path = self.ctx.router.find_path(self._pe_prefix, peer_pe_prefix)
-            return self.ctx.compute_drain_ns(path, self._credit_size_bytes)
+            path = self.ctx.router.find_path(self._pe_prefix, peer_pe_dma)
+            return self.ctx.compute_path_latency_ns(
+                path, self._credit_size_bytes,
+            )
         except Exception:
             return 0.0