test: measurement-only A-versus-window timing for shifted-MRT refined DFG

A (per-base read-and-sum per 80-step interval), W (window + step(80) +
one read) and W1 (window + 80 x step(1)) over equal simulated duration
from the same fresh state and warm-up, 3 interleaved repeats. Bitwise
populations and per-interval total impulses (documented engineering
tolerance) are compared outside timing; health checks untimed. Records
candidate source identity (revision, dirty flag, file hashes) and does not
use or modify frozen refined-DFG manifests/results.

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

docs/validation/refined_dfg_timing.md

@@ -42,3 +42,17 @@ Usage (root selects one verified idle GPU):
 
 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.
+
+## A versus observation-window ledger (`tests/validation/time_window_dfg.py`)
+
+Measurement-only comparison on the same shifted-MRT refined setup: A (per-base `step(1)` +
+`read_raw_exchange`, summed per 80-base-step interval), W (`start_exchange_window`, `step(80)`,
+one window read) and W1 (window with 80 x `step(1)`, isolating the read pattern from call
+batching). Same fresh state, warm-up and simulated duration; populations must be bitwise equal
+to the first A repeat and per-interval total impulses within the documented engineering
+tolerance, all outside timing. The candidate source is recorded (checkout revision, dirty flag,
+file hashes); frozen refined-DFG manifests/results are not used or changed.
+
+    PYTHONPATH=src python -m unittest tests/unit/test_time_window_dfg.py
+    python tests/validation/time_window_dfg.py --output <new> --plan
+    CUDA_VISIBLE_DEVICES=<gpu> python tests/validation/time_window_dfg.py --output <new> --max-wall-seconds <s>