test: add one projected x-gap-width shear contrast

docs/validation/taylor_green.md

@@ -459,3 +459,24 @@ What's kept and enforced:
   a 600 kB cap. If invalid it is dropped and an error is recorded, keeping
   the scalar receipt.
 - There are no new pass gates; this is a mechanism diagnostic only.
+
+### Single x-gap-width contrast (`wide_x_shear_n128`)
+
+This opt-in admits only Re100/N128/projected depth3 with `--probe-trace`.
+The outer boxes, three grid spacings, viscosities, initialization, five main
+sample times and trace windows are unchanged. L1→L2 parent-frame rectangles
+are `[(56,44,96,212),(160,60,200,228)]`: both x-normal middle gaps widen from
+6 to 12 root cells, while y gaps remain 6. Active leaves are
+`[7936,20352,53760]`, hops `[2048,4096,8192]`, work 540,016,640 active updates.
+This also moves the L2 x faces and reduces its volume; it is not a pure
+travel-distance experiment. No new qualification matrix, assessor, threshold
+or snapshot is added. Existing scalar screens and all failures stay visible.
+
+The same 24 targets and actual-center metadata are retained. The horizontal
+probes stop before the new inner face x = 28: they do not bracket that interface
+or sample its fine side. The first 8 horizontal L0/L1 probes keep identical
+centers, and the y-normal x = 38 transect is unchanged. Their temporal residuals
+can support a descriptive timing contrast, but cannot establish a crossing
+or reflection coefficient. Changed-level probes must not be treated as
+collocated samples. `rho-1` remains raw density deviation; do not substitute
+the 24-probe mean for the global pressure gauge.