test: add opt-in shifted FP32 Jet CV control

docs/validation/jet_circle_cv.md

@@ -143,3 +143,17 @@ Global stock-minus-raw mass balance is reported per root interval and summed per
 Planning envelope from the earlier52-second800-step pilot: s1 60–240s, s2 600–2400s, with conservative post-READY caps1200/5400s. Scale2 has4x cells and4x root observations over the same reference horizon; sparse saving reduces output, not all CPU/PCIe observation cost. These are preparation estimates, not measured timings. Root owns startup/compilation timeout and GPU serialization. No GPU/remote action was taken to prepare this option.
 
 Budget enforcement preflights the projected retained file sizes before replacing each snapshot/window report or the sample+metadata pair. If32MiB would be exceeded, staged replacements are removed, the previous verified sample/hash-metadata pair remains intact, and execution stops. Temporary compression files need additional short-lived disk space and are distinct from the32MiB retained-evidence cap. This protects budget failures; it is not a filesystem crash-recovery transaction. Both raw partition identities, total−wall−aperture and aperture−blowing−suction, are saved separately from stock/raw and CV residuals.
+
+## Optional shifted-FP32 representation control
+
+The dense-root campaign accepts `--shifted-fp32` as an explicit numerical-representation control. Without it, the historical unshifted FP32 default remains unchanged. Use a fresh output directory and append the flag to the reviewed dense command, for example:
+
+```bash
+CUDA_VISIBLE_DEVICES="$VERIFIED_IDLE_GPU" python tests/validation/run_jet_circle_cv.py --repo "$PWD" --output "$NEW_SHIFTED_DENSE_S1" --dense-root --steps 800 --scale 1 --shifted-fp32
+```
+
+Root owns the future GPU launch and scale selection. This option requires the bounded dense campaign; it does not authorize another run. Geometry, Jet profile/amplitude, initial physical seed, viscosity, time horizon, root sampling, sparse rays/fields, normalization, thresholds, budgets and prior failed gates are unchanged. The `case.ddf_shifting` flag and `case.storage_variant` receipt appear in the plan, READY, window report and sample metadata. Preserve the unshifted evidence in its original directory. Compare like-scale records at identical root endpoints, including CV mass/momentum, global stock-minus-raw balance, raw partitions and measured aperture flux. A reduced mass residual would support representation sensitivity; it would not establish physical convergence or prove that every residual is roundoff.
+
+Both production blocked steppers already support this representation: initialization takes physical populations and encodes `g=f-w` to FP32; public download returns physical `f` decoded in FP64. The runner deliberately retains its historical FP32-rounded physical equilibrium seed for BOTH variants rather than also changing initial precision. Encoding introduces a small additional rounding difference: maximum per-population change 1.035e-10 and density change 3.201e-10 at both scales for this seed. Thus the decoded starting states are not bit-identical. The residual initial density offset is therefore not removed. All existing measurement routines receive physical populations and apply no extra weight shift. Snapshots keep the existing primitive-field precision.
+
+The existing `BlockCurvedBoundary` optional-input adapter invokes the same shifted-aware BFL helper as the AMR boundary: wall density restores the unit background, raw mass uses reflected-minus-outgoing (opposite weights cancel), and reaction restores twice the direction weight. No host correction is applied to raw rows. The shifted branch selects `compute_feq_from_momentum` rather than unshifted `compute_feq`, so this control changes existing numerical algebra as well as storage representation. It cannot isolate storage precision alone or identify collision as the sole cause. Block collision preserves the density perturbation in its shifted branch; blocked halo copies and AMR transfers already support shifted storage. This option changes no solver/kernel/API or source manifest. Existing shifted-storage/Jet host tests and the new runner codec tests are CPU evidence only; numerical GPU behavior remains to be checked by the reviewed control.