LlvmUnit

`SUP-3`'s lane — the same SPIR-V, lowered to LLVM and run natively.

In the namespace eval::internal. Declared in src/libs/libeval/eval/SpirvLane.hpp.

Detailed description

Built from the bytes a SpirvUnit already produced, so the two lanes are provably consuming the **same module** rather than two compilations of the same text. That is what makes their disagreement mean something: any difference is in the lowering, because there is nothing else left to differ.

Compiled once and called many times, for the same reason `SpirvUnit` is: the harness evaluates 512 sample points per record, and clang-less code generation plus a link per point would make the lane a measurement of the toolchain.