sim: report correct kernel latency under enable_data=False

Three coupled fixes so enable_data=False now reports byte-identical
kernel-body sim latency (max(t_end) - min(t_start) over op_log) as
enable_data=True, and skips setup-write sim events that were pure
wall-clock overhead.

Before this change, at Case 4 (Cube-SP x PE-SP) decode, S_kv=8K:
  enable_data=False  -> latency_ns = 0  (op_log empty)
  enable_data=True   -> latency_ns = 30.646 us  (correct)

After:
  enable_data=False  -> latency_ns = 30.646 us  (byte-equal)
  enable_data=True   -> latency_ns = 30.646 us  (unchanged)

engine.py: always create OpLogger. Previously OpLogger was gated on
enable_data=True together with MemoryStore, so op_log stayed empty when
data mode was off. Decouple: MemoryStore is gated (Phase 2 DataExecutor
still needs it) but OpLogger runs unconditionally, receiving
memory_store=None when data mode is off. OpLogger already guards its
arr.copy() snapshot paths on `if self._memory_store is not None`.

pe_cpu.py: always use the ADR-0020 greenlet execution path. The
if store is not None: _execute_greenlet(); else: _execute_legacy branch
gated the *execution model* on data-mode presence. The legacy
command-list path predates ADR-0020 and doesn't route IpcqSendCmd
through PE_IPCQ (it goes to PE_SCHEDULER instead), so all 189 Case-4
ipcq_copy fabric transfers were silently no-op'd in that mode. Forcing
greenlet gives identical sim behavior in both modes. KernelRunner
already guards its store reads on `self._store is not None`.

context.py: gate setup MemoryWriteMsg on memory_store presence. Under
enable_data=False there is no MemoryStore to populate and no Phase 2
replay, so the per-shard sim events for Q/K/V deploy are pure wall-clock
overhead with no effect on reported kernel latency (Yangwook's max-min
formula excludes pre-kernel ops). Handle count for Case 4 at S_kv=8K
drops 229 -> 37 under enable_data=False.

Verified:
- tests/attention/test_milestone_gqa_decode_long_ctx_4cases.py: 18/18 pass
- Parity probe: enable_data=False/True both report 30.646 us kernel sim
  time, 1649 op_log records, 189 ipcq_copy events

Known follow-up: pe_cpu._execute_legacy is now dead code but left in
place; separate cleanup once we confirm no downstream caller.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/kernbench/components/builtin/pe_cpu.py

@@ -117,17 +117,13 @@ class PeCpuComponent(ComponentBase):
         pe_exec_start = env.now
         scheduler_id = f"{self._pe_prefix}.pe_scheduler"
 
-        # Choose execution mode: greenlet (ADR-0020) or legacy command-list
+        # ADR-0020 greenlet execution — always used so IPCQ / ring credits /
+        # fabric-transfer sim events fire regardless of data-mode. KernelRunner
+        # guards its store reads on ``self._store is not None``.
         store = getattr(self.ctx, "memory_store", None) if self.ctx else None
-
-        if store is not None:
-            composite_results = yield from self._execute_greenlet(
-                env, kernel_fn, kernel_args, num_programs, scheduler_id, store,
-            )
-        else:
-            composite_results = yield from self._execute_legacy(
-                env, kernel_fn, kernel_args, num_programs, scheduler_id,
-            )
+        composite_results = yield from self._execute_greenlet(
+            env, kernel_fn, kernel_args, num_programs, scheduler_id, store,
+        )
 
         # Record PE-internal execution time
         txn.result_data["pe_exec_ns"] = env.now - pe_exec_start