test: adopt native-validated 24-target probe layout

Horizontal y=56 and vertical x=38 through box 1, normal coordinates
[12.5..26.125] (root128 physical). Nearest-ACTIVE mapping with
deterministic tie rule (distance, finer level, smaller j, smaller i),
duplicate-cell rejection, stored centers and displacements. Replaces my
earlier offsets, which mapped two target pairs to the same U128 cell.
U512 spacing 128/512 and root step r -> U512 step 4r unchanged.
Tests: 24 unique cells per case, nested 3 L0 + 5 L1 + 4 L2 per transect,
U128 max displacement 0.625.

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

docs/validation/taylor_green.md

@@ -421,12 +421,18 @@ Labels:
 
 ### Signed interface-probe traces (opt-in `--probe-trace`, mechanism diagnostic)
 
-`shear_probes.py` (pinned) defines 24 fixed points on two interface-normal
-transects, all at least 8 root cells from any box corner:
-- V: y = 64.1, crossing the vertical faces x = 16 (L0/L1) and x = 22 (L1/L2);
-- H: x = 90.1, crossing the horizontal faces y = 24 and y = 30;
-- offsets from the outer face: −3.9, −1.9, −0.4, 0.1, 0.6, 2.1, 4.1, 5.6, 6.1,
-  6.6, 8.1, 10.1.
+`shear_probes.py` (pinned) uses native's validated 24-target layout through
+box 1, in root-128 physical coordinates:
+- a horizontal line y = 56, crossing the vertical faces x = 16 and 22;
+- a vertical line x = 38, crossing the horizontal faces y = 16 and 22;
+- normal coordinates 12.5, 14.5, 15.5, 16.25, 17.25, 19.25, 20.75, 21.75,
+  22.125, 23.125, 24.125, 26.125.
+
+Each target maps to the nearest ACTIVE leaf center. The deterministic tie
+rule is: distance, then finer level, then smaller j, then smaller i. This
+gives 24 unique cells in every case, and nested has 3 L0 + 5 L1 + 4 L2 per
+transect. Maximum displacement from target to center is 0.625 root cells in
+U128 (0.177 in U512).
 
 Allowed cases:
 - nested U128, nested U512, nested projected depth 3;
@@ -435,9 +441,8 @@ Allowed cases:
 The option can't be combined with `--final-snapshot`.
 
 Sampling:
-- Each point is read from the ACTIVE leaf cell containing it. The receipt
-  stores the leaf level, `i`/`j`, and the actual cell center, all in
-  root-physical units (N=128).
+- The receipt stores each probe's leaf level, `i`/`j`, actual cell center
+  and displacement from its target, all in root-physical units (N=128).
 - Grids differ between cases, so centers are *not* collocated. Analytic
   fields are evaluated at each actual center (`shear_probes.residuals`), and
   comparisons across cases are between residuals at nearby but distinct