v0.7.0 — the movement was the cost, not the arithmetic

Two passes folded away, bit for bit

Synthesize2D_97_WithDims is 48% of the heaviest page in the corpus, and the
arithmetic is the small part of it:

1.12 s flat in Synthesize2D_97_WithDims	the gather and scatter
0.45 s liftstep97_c_f64_neon	the lifting, NEON assembly
0.40 s BaseInterleave_fallback_Float64	the interleave, pure Go
0.30 s runtime.memmove	the two copies
0.23 s scaleslice_c_f64_neon	the scaling, NEON assembly

1.82 s of moving data against 0.68 s of computing it. Two passes carry no
arithmetic of their own and are now folded into the passes either side: the
scaling into the read-out of each subband (a copy then *= K is the same
single multiply as = data[i] * K), and the interleave, in the vertical pass,
into the scatter.

Measured

min-of-5, two interleaved rounds, on the three heaviest JPEG 2000 pages, against
a ±1% noise floor:

document	before	after
b31820797.pdf	786.3 ms	721.2 ms	−7.7%
bulletinchicagov48nchicg.pdf	657.7 ms	636.7 ms	−4.4%
irrigationrequir1340fort.pdf	404.7 ms	388.5 ms	−3.4%

Unchanged output

All 3 215 corpus documents render byte-identical. TestFewerPassesIsBitIdentical
compares the new synthesis against the shape it had before in exact float64
bits, over twelve geometries including odd sizes and both parities — and it runs
on all six architectures, so the fold is exact on big-endian s390x and on 32-bit
too, where the lifting has no assembly path at all.

Two things measured and NOT done

• A fused kernel in scalar Go (one call over six passes instead of seven calls
  over nine) is 1.5× to 1.8× slower. The library's lifting is NEON assembly
  with FMA and losing it costs more than the passes save. The same FMA makes that
  path unreproducible in Go: the identical expressions differ by 1 to 16 ulp.
• BaseInterleave runs the pure-Go fallback on arm64 — the generated NEON
  variant exists and is never assigned. Benchmarked head to head, it makes no
  difference: both run at 25–28 GB/s, which is memory bandwidth. An interleave is
  pure movement; the answer is to remove the pass, not vectorise it.

Also takes go-highway v0.0.12, eight versions newer than what this package was
pinned to. Nothing was updating it.