test: depth-2 twin of the nested shear preset (twin_shear_n128)

Test-only opt-in. New shear_twin.py: same two L0->L1 boxes as
shear_nested, no L2; leaves [7936,33792], hops [2048,4096], work
154664960. Four fresh cases U128/U256/constant R128 d2/projected R128 d2;
physics is the unchanged depth-2/2N shear_wave.assess. run_case:
--diagnostic twin_shear_n128 with shear_twin.py+shear_nested.py pins, no
snapshot. assess_shear_wave: nested contract made preset-generic (PRESETS)
with nested behaviour unchanged. Labels: twin vs nested = effect of adding
this nested refinement (not abstract depth). shear_wave.py, shear_nested.py
and oracle.py byte-identical.

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

docs/validation/taylor_green.md

@@ -385,3 +385,36 @@ Assessment: `assess_shear_wave.py U128.json U512.json R128c.json R128p.json
   identities and verdicts are untouched.
 - This preset changes placement and refined volume as well as depth. It
   gives no isolated cost of an extra level and no resolution trend.
+
+### Depth-2 twin of the nested preset (opt-in `twin_shear_n128`)
+
+`shear_twin.py` uses the same two L0→L1 boxes as `shear_nested.py`, with no
+L2. Leaves are `[7936, 33792]`, hops `[2048, 4096]`, and work is 154664960 per
+refined case (611319808 for all four).
+
+The campaign is four fresh runs:
+`run_case.py --flow shear_wave --diagnostic twin_shear_n128 --re 100` with
+- `--n 128 --depth 1`
+- `--n 256 --depth 1` (the finest control is 2N)
+- `--n 128 --depth 2`
+- `--n 128 --depth 2 --explosion projected_linear`
+
+There is no snapshot option. Older exact controls serve only as consistency
+checks; they are not reused as references.
+
+Assessment uses the same preset contract as the nested preset, through
+`assess_shear_wave.py ... --diagnostic twin_shear_n128`:
+- the physics is the unchanged depth-2 / 2N `shear_wave.assess`;
+- receipts must pin `shear_twin.py`, `shear_nested.py` and `shear_wave.py`;
+- `shear_nested.py` stays byte-identical (fa6cfe87…), so the 335624c nested
+  receipts still validate.
+
+Labels:
+- *Twin vs nested* is the effect of adding this nested L2 refinement.
+  Removing L2 also changes the finest spacing and refined volume, so it does
+  not isolate abstract depth.
+- *Twin vs the single-box depth-2 runs* is a placement/geometry contrast at
+  equal depth.
+- Neither contrast is additive by construction.
+- The thin-band/acoustic explanation for the nested L1 pressure is a
+  hypothesis, not established.