# Experimental projected G explosion

`FixedPatchStepper(..., explosion="projected_linear")` changes only the existing
G-to-G explosion reconstruction. The omitted/default `"constant"` selector retains
the same kernel, launch arguments and device allocations. Projected mode adds two
compact tables per adjacent pair; recursion, collision, BFL, restriction, buffer
ownership and synchronized publication are unchanged. `remap_finest` preserves it.

Runtime eligibility is periodic-root, fully blocked D2Q9/SRT/FP32, body/Jet-free,
without LES; the fixed-patch backend has no forcing interface. Shifted and unshifted
storage use the same reconstruction. One generic builder handles scalar windows,
unique union rows and all adjacent hierarchy pairs without depth/rectangle caps.
Donors are original explosion donors intersected with owned, uncovered ACTIVE
cells. An axis uses centered differences only if both neighbors are legal;
otherwise both addresses repeat the center. Nested layout periodicity is explicit.
No covered cells, holes, inactive cells or stale halo copies enter the stencil.

The 33-line kernel is unchanged from our `519473a` experiment and retains its
mathematical attribution. Host tests cover union decomposition, concavity, narrow
gaps, grazing packet paths, offsets, sparse/nested owner addresses, three/four-level
source timing, unchanged constant dispatch, remapping and shifted/unshifted actual
kernel arithmetic compiled as host C++. These are not CUDA or flow validation.

## Reviewed bounded evidence

The `1aa8110` CUDA gate, with production exactly `a135252`, passed exact-dev
omitted/default and explicit-constant equivalence, per-population reconstruction,
scalar/union/four-level scheduling, alternate layouts, and changed remapping in
both storage modes. Operator maximum error was 2.98023224e-8; the eight-step
concave replay maximum was 1.58037614e-7. Whole-step mass/momentum remain
finite diagnostics, not an FP32 conservation certificate. Reviewed evidence:
`7aa66ff0542175092753ea099f40b903ece0527e`.

The fresh four-case `2cf9853` N128/256 Taylor–Green repeat retained all `11901a7`
settings and gates. All 20 main scalar rows and 260 early-pressure samples matched
the older receipts exactly; this is not field-array equality. Projected worst
velocity was 0.001995299260844 versus 0.002 (only 0.235% margin), pressure
0.012157174904932 and energy 0.001184362645162. Constant velocity
0.004418076615223 failed its gate, so the aggregate verdict remains false.
The projected early sampled pressure peak was 0.021621453573334; it remains a
descriptive sampled maximum. Reviewed evidence: `90849a55cad4a0da9b72747da7bfe6eb75962a44`.

The old N64 projected failures remain relevant. These results support only this
bounded plumbing gate and short periodic Re100 smooth-decay experiment. They do
not establish union/multilevel physics accuracy, convergence, long-time stability,
body/Jet, boundary, forcing, MRT or LES suitability. Constant remains the default.

## Separate LES source profile

The two older LES source manifests and the historical default are unchanged and
correctly reject this different source tree. The existing LES runner now offers
`--experiment a135252`, pinning all 101 actual committed corrected source files.
Its explicit baseline-header override describes only a synthetic isolated reverse
control, never the committed a135252 header. The unchanged corrected-only uniform
LES regression (Cs=0/.16/1) is the remaining integration gate; no LES result is
inherited from the projected evidence. The separate historical jet-CV `channel.py`
pin mismatch predates this port and is not addressed here.