Add matched diffusive resolution cases for pinball response

docs/steady_pinball_fiv.md

@@ -102,3 +102,49 @@ per-case wall cap and GPU headroom check bound resource use. Preserve partial ro
 do not add damping, clip trajectories, filter forces, or relax any stop. Each
 case adds72MB of scalar history, no full fields. The separate Legacy/modern
 baseline diagnosis does not change this FSI protocol.
+
+## Matched D40 resolution comparison (predeclared 2026-10-31)
+
+The first finer case is M2, with exactly the same mass ratio2, reduced velocity5,
+zero structural damping, initial rest state and physical geometry as the retained
+D20 M2. Targeting a development response of0.5--1D does not imply this case will
+reach it. Do not change the structure within this comparison or discard a smaller
+response. D60 is a supported scaling choice, not an automatically scheduled run.
+
+For s=D/20 use dx'=dx/s, dt'=dt/s^2, U'=.01/s, nu'=.004, rho'=1,
+fn'=fn/s^2, M'=s^2 M and K'=K/s^2. The same physical force per unit span
+has F'_lat=F_lat/s. For D40 this gives U=.005, fn=.000025,
+M=7539.82237, K=.000186037660, Re_D=50, and Tn=40000 steps.
+The y walls are at .5 and ny-1.5, while x inlet/outlet source nodes are at0
+and nx-1. Therefore map x'=s*x and y'=.5+s*(y-.5), with
+nx'=s*(1280-1)+1, ny'=s*(512-2)+2. D40 uses2559x1022 and centres
+(1200,510.5),(1252,540.5),(1252,480.5); every radius, probe and envelope scales
+with D. The parabolic profile retains the same normalized wall coordinate.
+This preserves the existing ghost-source boundary convention, without asserting
+that it is an exact continuum open-boundary location.
+
+Warmup=2048000 and response=2400000 native steps at D40: the SAME256 and300
+D/U durations as D20, including60 natural periods after release. Compare the
+six50-D/U blocks and the common final100-D/U window. Report max absolute y/D,
+half-range, RMS, frequency and drift; peak-to-peak is not amplitude. No settling
+claim follows from completion. Keep raw individual CME/Wen/node terms and all
+crossing/fallback counters. Normalize net force by .5*rho*U^2*D in each case.
+Report native crossing/non-crossing force increments separately, plus D40
+four-step block means aligned with D20 intervals (diagnostic only, never replace
+the raw spikes). Report final-window high-frequency variance above f*D/U=1,
+using demeaned FFT Parseval weights, with the same physical cutoff for both cases.
+Different motion histories confound spike-size comparisons; this is simultaneous
+space/time/Mach refinement, not proof of a unique coarse-grid cause.
+
+Only one full D40 M2 is initially scheduled, after CPU checks and a32+32-step
+smoke. Stop on the unchanged geometry/speed/nonfinite/no_donor checks or a7200s
+whole-case wall cap; save completed rows and status. The velocity cap scales to
+.025 so its physical bound is unchanged. No force filtering, trajectory clipping,
+automatic parameter search or extension. Scalar history is288MB; no full fields
+or DDF snapshots are written. Two FP32 population buffers require about188MiB,
+before flags, moving tables and context overhead; measure actual peak usage in
+the smoke and full run. Keep all D20 outcomes. No production source changes.
+
+Commands (one GPU and serial compilation, existing environment):
+`python research/steady_pinball/run.py --case M2 --D 40 --smoke --out <new-smoke-dir>`
+then `python research/steady_pinball/run.py --case M2 --D 40 --wall-seconds 7200 --out <new-full-dir>`.