gqa(decode-4cases): wire latency + engine occupancy into sweep.json; add comparative plot script (5C.F)

Bench changes:
  - new _end_to_end_ns(op_log) and _engine_occupancy_ns(op_log) helpers
    in milestone_gqa_decode_long_ctx_4cases.py (mirror paper_gqa_latency.py)
  - _run_panel return dict now carries latency_ns + engine_occupancy_ns
    alongside op_log_summary, so sweep.json is the single source of truth
    for the comparative figures

Plot script:
  - new scripts/paper/paper_plot_gqa_decode_long_ctx_4cases.py reads
    sweep.json and emits 3 PNGs to docs/report/1H-codesign-paper/figures/:
      gqa_decode_long_ctx_4cases_latency.png   (end-to-end latency / case)
      gqa_decode_long_ctx_4cases_traffic.png   (ipcq/dma op counts / case)
      gqa_decode_long_ctx_4cases_memory.png    (KV bytes per cube / case)

Test changes:
  - 2 new tests verifying the helpers + _run_panel dict shape
  - lower smoke S_kv from 8192 -> 2048 (4x faster Case 2; assertions
    are S_kv-independent; one fold-loop iteration preserved)

18/18 tests pass in ~4 min.

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

src/kernbench/benches/milestone_gqa_decode_long_ctx_4cases.py

@@ -258,6 +258,33 @@ def _make_bench_fn(panel: str):
     return _bench_fn
 
 
+# ── Panel metrics (sweep.json carries these for the comparative plot) ─
+
+
+_ENGINE_SUFFIXES = (
+    "pe_gemm", "pe_math", "pe_dma", "pe_fetch_store", "pe_ipcq", "pe_cpu",
+)
+
+
+def _end_to_end_ns(op_log) -> float:
+    """End-to-end window: ``max(t_end) - min(t_start)`` over all records."""
+    if not op_log:
+        return 0.0
+    return max(r.t_end for r in op_log) - min(r.t_start for r in op_log)
+
+
+def _engine_occupancy_ns(op_log) -> dict[str, float]:
+    """Per-engine summed occupancy (component_id suffix match)."""
+    return {
+        eng: sum(
+            r.t_end - r.t_start
+            for r in op_log
+            if r.component_id.endswith("." + eng)
+        )
+        for eng in _ENGINE_SUFFIXES
+    }
+
+
 def _run_panel(panel: str, topology: str) -> dict:
     from kernbench.runtime_api.bench_runner import run_bench
     from kernbench.runtime_api.types import resolve_device
@@ -278,11 +305,14 @@ def _run_panel(panel: str, topology: str) -> dict:
             f"{result.completion}"
         )
     kind, params = _PANEL_DISPATCH[panel]
+    op_log = result.engine.op_log
     return {
         "panel": panel,
         "kind": kind,
         **params,
-        "op_log_summary": _summarize_op_log(result.engine.op_log),
+        "op_log_summary": _summarize_op_log(op_log),
+        "latency_ns": _end_to_end_ns(op_log),
+        "engine_occupancy_ns": _engine_occupancy_ns(op_log),
     }