research: reuse quadrature and clarify snapshot and VIV coordinates

docs/validation/viv_re150_predeclare.md

@@ -29,10 +29,17 @@ Claude, Grok and Astra at 1abfe954; implemented in research/viv_re150/.
 
 - D = 40, MRT, dense double buffer; U_lat = .04, nu = U D / 150 = .010667,
   tau = .532.
-- Box [0, 75 D] × [0, 60 D] (3000 × 2400) with the cylinder centre at
-  (30 D, 30 D): 30 D from the inlet, 45 D from the outlet, centred in y
-  (Kolahdouz §4.1.1). Qin's own box ([0, 40 D] × [0, 20 D], centre (10 D, 10 D))
-  is the target paper's, not ours.
+- Nominal 75 D × 60 D lattice allocation (3000 × 2400), based on the
+  Kolahdouz §4.1.1 layout. Coordinate clarification of the existing run:
+  the driver uses `(30 D, (ny - 1)/2)` = `(1200, 1199.5)` in lattice
+  coordinates, or `(30 D, 29.9875 D)`. The earlier `(30 D, 30 D)` wording
+  described the nominal box, not the exact executed centre.
+  Free-slip reflection planes are at `y = .5` and `y = ny - 1.5 = 2398.5`;
+  their separation is 2398 cells = 59.95 D, with the centre exactly midway,
+  29.975 D from each wall. Node counts are not physical wall-to-wall lengths.
+  This corrects the description, without changing the historical geometry
+  or results. Qin's own box ([0, 40 D] × [0, 20 D], centre (10 D, 10 D)) is
+  the target paper's, not ours.
 - Boundaries: uniform Zou–He inlet, NEQ outlet (rho = 1, backflow clamp),
   free-slip y walls. Kolahdouz uses a traction outlet: declared mismatch.
 - Qin's own VIV box (40 D × 20 D) is not used; a smaller box would be a