Speed up regression: 25min → 6min (test matrix + DataExecutor cleanup)

Test matrix restructure:
- 256-rank full-system ring runs only ONCE (marked pytest.mark.slow)
  instead of 7× across matrix + perf tests. Cross-SIP routing is
  verified by the single run; buffer variants (tcm/hbm/sram) are
  tested at 8-rank where they finish in <0.5s.
- Performance tests use 8-rank instead of 256-rank.
- `pytest -m "not slow"` completes in ~2.5min (local dev).
- Full suite including slow: ~6min (CI).

DataExecutor optimization:
- Remove ThreadPoolExecutor from DataExecutor.run(). Same-t_start
  groups are almost always size 1, so the thread pool creation and
  dispatch overhead dominated. Simple sequential loop is faster.
- Skip dma_read ops at the loop level (they are always no-ops in
  Phase 2 but were dispatched through _execute_op → _execute_memory).
- Remove redundant CLI Phase 2 re-execution: engine._flush_data_phase
  already replays during engine.wait(); the CLI now only prints the
  diagnostic summary without re-running DataExecutor.

502 tests pass. Wall time: 25m30s → 5m43s (full), 2m28s (no slow).

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

src/kernbench/cli/main.py

@@ -72,15 +72,12 @@ def cmd_run(args) -> int:
         print(format_report(result.traces, title=args.bench, spec=spec))
     print(result.summary_text())
 
-    # Phase 2: data execution (ADR-0020)
+    # Phase 2 diagnostic summary (ADR-0020). The actual Phase 2 replay
+    # already runs inside engine.wait() → _flush_data_phase(). We only
+    # print the summary here; no redundant re-execution.
     if verify_data and result.engine is not None:
-        from kernbench.sim_engine.data_executor import DataExecutor
-
         op_log = result.engine.op_log
-        store = result.engine.memory_store
-        if op_log and store is not None:
-            executor = DataExecutor(op_log, store)
-            executor.run()
+        if op_log:
             n_gemm = sum(1 for r in op_log if r.op_kind == "gemm")
             n_math = sum(1 for r in op_log if r.op_kind == "math")
             print(f"[data] Phase 2 complete: {len(op_log)} ops ({n_gemm} gemm, {n_math} math)")