transformers: route fused ScaledMaskedSoftmax through an accurate vectorized exp

[czoli1976]

What

ScaledMaskedSoftmax::eval hard-coded SoftmaxExp::Libc, so the fused attention softmax always ran scalar libm expf and never reached the linalg SIMD softmax kernels — a real perf gap. This PR closes that gap without sacrificing accuracy, by adding an accurate vectorizable exp and routing the fused softmax through it.

History: this PR originally just flipped Libc → FastCompact. CI (and a local full-suite run) showed that was wrong on two counts, so the approach was reworked — see below.

Why not FastCompact

Switching the fused softmax to the existing SoftmaxExp::FastCompact (Schraudolph approximation) fails the scaled_masked_softmax / sdpa proptests two independent ways:

1. Precision. FastCompact's exp is ~0.5% off true softmax — outside the suite's Approximate tolerance (f32 rtol 5e-4), producing 30%+ outliers. (The existing softmax_l2 frame test only compares FastCompact against itself, so it never caught this.)
2. Fully-masked rows → NaN mismatch. On an all--inf row the FastCompact kernel pads the SIMD tail with f32::MIN and computes exp(f32::MIN - f32::MIN) ≈ 1, so the row sums to a nonzero value and yields a finite 0 where the scalar libc path and the numpy reference both yield NaN (0 * 1/0).

This mirrors what ggml/llama.cpp concluded (ggml-org/llama.cpp#7154): keep an accurate vectorized expf for softmax, reserve fast-approx exp for error-tolerant ops.

What changed

• linalg: accurate_exp_f32 — a Cephes-style range-reduced exp (Cody-Waite ln2 split + degree-6 polynomial + 2^n by exponent construction). Measured max rel error ~1.9e-6 vs libc over the softmax domain [0, -60]. exp(0)==1 and exp(-inf)==0 exactly; deep underflow flushes to 0; NaN propagates.
• linalg: SSoftMaxL2Accurate / HSoftMaxL2Accurate map-reduce kernels, exposed as softmax2_accurate_{f32,f16}. They pad the SIMD tail with -inf (not f32::MIN), so masked/padding lanes contribute exactly 0 and a fully-masked row sums to 0 → NaN, matching libc and the reference.
• core: new SoftmaxExp::Accurate variant + dispatch (f32/f16).
• nnef: exp = "accurate" de/serialization round-trip.
• transformers: ScaledMaskedSoftmax::eval uses SoftmaxExp::Accurate.

Libc remains the default everywhere; FastCompact is untouched. This adds a third, accurate-but-vectorized option and points fused attention at it.

Tests

• New linalg tests validate accurate_exp_f32 against libc (not against itself) and cover the fully-masked degenerate row (sum == 0).
• scaled_masked_softmax + sdpa proptests (f16/f32 × raw/decluttered/optimized) pass on native and wasm32-wasip1 (the job that originally failed).
• Full tract-linalg suite green; tract-core / nnef / transformers green; cargo fmt clean; no new clippy warnings on touched files.

🤖 Generated with Claude Code

[czoli1976]

taking it back, correctness failure to investigate