Dual-execution parity test corpus

Goal

Pin the guarantee and the parity contract.

Implementation Sketch

Positive pins (every guaranteed-eligible pattern vectorizes and matches scalar), negative pins (reorder-unsafe reductions + panic-capable element bodies stay scalar or first-panic-preserving by default), and the #fast_math both-backend determinism pin. Debug + release; eval vs LLVM identical for values AND panics.

Spec References

Proposal §6 parity; CLAUDE.md dual-execution parity.

Work Items

• Positive pins: each guaranteed-eligible pattern (element-wise map/zip, reorder-safe reduction) vectorizes and produces the scalar-identical result.
• Negative pins: reorder-unsafe reductions (checked int +/, FP +/) and panic-capable element bodies stay scalar (or first-panic-preserving) by default; a #fast_math pin asserts both backends agree deterministically.
• Dual-execution parity across the corpus: eval and LLVM identical for values AND panics, debug and release.