test: apply timing harness review caveats

Explicit stream synchronization before/after each timed loop (refined
FixedPatchStepper stream, uniform Simulation stream, in their contexts);
required --max-wall-seconds outer timeout checked between variants and in
per-step loops; untimed finite leaf/fluid health checks; force notes for
B/C/E' (no host reads; final base interval only; whole-window average with
different summation order). Parity stays A/B/C vs A and E/E' vs E only.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/validation/refined_dfg_timing.md

@@ -7,18 +7,21 @@ physics-gate or production change; no force or accuracy result is produced.
 | Variant | Loop | Force history |
 |---|---|---|
 | A | refined `step(1)` + `read_raw_exchange` per base step (as the runner) | valid |
-| B | refined `step(1)` only | timing-only |
-| C | refined single `step(N)` | timing-only |
+| B | refined `step(1)` only, no host reads (`observe_raw_exchange` stays on) | timing-only |
+| C | refined single `step(N)` | timing-only; ledger keeps the final base interval only |
 | E | uniform `sim.run(<=100)` + `read_force` (as the runner) | valid |
-| E' | uniform single `sim.run(2N)` | timing-only |
+| E' | uniform single `sim.run(2N)` | timing-only; at most a whole-window average, different summation order |
 
 - Equal simulated duration: N refined base steps vs 2N uniform D80 steps.
 - Every repeat starts from the same fresh state (refined `initialize`, uniform `snapshot/restore`)
   and the same warm-up; construction/JIT, warm-up, downloads and parity checks are untimed.
-- `step()` synchronizes after each base interval and `Simulation.run()` before returning, so the
-  wall timer (`time.perf_counter`) brackets completed GPU work. No CUDA events are used.
-- Final populations and NaN masks must be bitwise identical to the first A (refined) or E
-  (uniform) repeat; any difference aborts.
+- The actual stepping stream is synchronized explicitly before and after each timed loop; the
+  wall timer (`time.perf_counter`) is the only source. No CUDA-event span is called kernel time.
+- `--max-wall-seconds` is required for GPU runs (outer timeout, checked between variants and
+  every 100 steps in per-step loops).
+- Final populations and NaN masks must be bitwise identical to the first A repeat (A/B/C) or the
+  first E repeat (E/E'); refined and uniform grids are never compared. Leaf/fluid populations
+  must be finite. All parity/health checks are untimed.
 - Cases run sequentially (uniform, then refined) in one process with startup serialized;
   variants are interleaved within each repeat. A-B and B-C are measured comparative
   differences under GPU variability, not exact causal decompositions.
@@ -27,7 +30,7 @@ Usage (root selects one verified idle GPU):
 
     PYTHONPATH=src python -m unittest tests/unit/test_time_refined_dfg.py
     python tests/validation/time_refined_dfg.py --output <new> --plan
-    CUDA_VISIBLE_DEVICES=<gpu> python tests/validation/time_refined_dfg.py --output <new>
+    CUDA_VISIBLE_DEVICES=<gpu> python tests/validation/time_refined_dfg.py --output <new> --max-wall-seconds <s>
 
 Defaults: 2000 base steps, 200 warm-up base steps, 3 repeats. Profiler capture and the
 `observe_raw_exchange=False` variant are deferred until these measurements exist.