test: measurement-only refined DFG MRT timing harness

Wall-time A/B/C/E/E' variants over equal simulated duration, reusing
run_refined_dfg setup functions. Fresh state and identical warm-up per
repeat, 3 interleaved repeats, construction/JIT, warm-up and bitwise
population/NaN-mask parity checks outside timed regions. B, C and E' are
labelled timing-only (no valid force history). No numerical or
production change.

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

docs/validation/refined_dfg_timing.md

@@ -0,0 +1,33 @@
+# Refined DFG MRT timing harness (measurement only)
+
+`tests/validation/time_refined_dfg.py` times the matched MRT refined/uniform DFG Re100 setup of
+`run_refined_dfg.py` (same configuration, geometry and initial state functions). No numerical,
+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 |
+| E | uniform `sim.run(<=100)` + `read_force` (as the runner) | valid |
+| E' | uniform single `sim.run(2N)` | timing-only |
+
+- 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.
+- 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.
+
+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>
+
+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.