research: Dütsch driver — exact-point body mask, periodic geometry from step mod T

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

research/duetsch_re100_kc5/run.py

@@ -70,11 +70,12 @@ def main():
                 fields = sim.get_macroscopic()
                 fluid = (sim.get_flags().reshape(n, n) & 1) != 0
                 ux, uy = (np.asarray(fields[k]).reshape(n, n) for k in ("ux", "uy"))
+                centre = (xm + p.position(s, T, A), ym)  # analytic centre after the step
                 for xs in p.STATIONS:
                     for x2 in p.X2:
                         x, y = xm + xs * D, ym + x2 * D
-                        u, wu, wsum = p.bilinear_fluid(ux, fluid, x, y)
-                        v, _, _ = p.bilinear_fluid(uy, fluid, x, y)
+                        u, wu, wsum = p.sample(ux, fluid, x, y, centre, R)
+                        v, _, _ = p.sample(uy, fluid, x, y, centre, R)
                         samples.append((s, p.cycle(s, T), ph, xs, x2,
                                         math.nan if u is None else u, math.nan if v is None else v,
                                         wsum, json.dumps(wu)))